RE: no change observed in read latency after switching from EBS to SSD storage
Chris, I agree that reading 250k row is a bit excessive and that breaking up the partition would help reduce the query time. That part is well understood. The part that we can't figure out is why read time did not change when we switched from a slow Network Attached Storage (AWS EBS) to local SSD. One possibility is that the read is not bound by disk i/o, but it is not cpu or memory bound either. So where is it spending all that time? Another possibility is that even though it is returning only 193311 cells, C* reads the entire partition, which may have a lot more cells. But even in that case reading from a local SSD should have been a lot faster than reading from non-provisioned EBS. Mohammed From: Chris Lohfink [mailto:clohf...@blackbirdit.com] Sent: Wednesday, September 17, 2014 7:17 PM To: user@cassandra.apache.org Subject: Re: no change observed in read latency after switching from EBS to SSD storage Read 193311 live and 0 tombstoned cells is your killer. returning 250k rows is a bit excessive, you should really page this in smaller chunks, what client are you using to access the data? This partition (a, b, c, d, e, f) may be too large as well (can check partition max size from output of nodetool cfstats), may be worth including g to break it up more - but I dont know enough about your data model. --- Chris Lohfink On Sep 17, 2014, at 4:53 PM, Mohammed Guller moham...@glassbeam.commailto:moham...@glassbeam.com wrote: Thank you all for your responses. Alex - Instance (ephemeral) SSD Ben - the query reads data from just one partition. If disk i/o is the bottleneck, then in theory, if reading from EBS takes 10 seconds, then it should take lot less when reading the same amount of data from local SSD. My question is not about why it is taking 10 seconds, but why is the read time same for both EBS (network attached storage) and local SSD? Tony - if the data was cached in memory, then a read should not take 10 seconds just for 20MB data Rob - Here is the schema, query, and trace. I masked the actual column names to protect the innocents :) create table dummy( a varchar, b varchar, c varchar, d varchar, e varchar, f varchar, g varchar, h timestamp, i int, non_key1 varchar, ... non_keyN varchar, PRIMARY KEY ((a, b, c, d, e, f), g, h, i) ) WITH CLUSTERING ORDER BY (g ASC, h DESC, i ASC) SELECT h, non_key100, non_key200 FROM dummy WHERE a='' AND b='bb' AND c='ccc' AND d='dd' AND e='' AND f='ff' AND g='g'AND h ='2014-09-10T00:00:00' AND h='2014-09-10T23:40:41'; The above query returns around 250,000 CQL rows. cqlsh trace: activity | timestamp| source | source_elapsed - execute_cql3_query | 21:57:16,830 | 10.10.100.5 | 0 Parsing query; | 21:57:16,830 | 10.10.100.5 |673 Preparing statement | 21:57:16,831 | 10.10.100.5 | 1602 Executing single-partition query on event | 21:57:16,845 | 10.10.100.5 | 14871 Acquiring sstable references | 21:57:16,845 | 10.10.100.5 | 14896 Merging memtable tombstones | 21:57:16,845 | 10.10.100.5 | 14954 Bloom filter allows skipping sstable 1049 | 21:57:16,845 | 10.10.100.5 | 15090 Bloom filter allows skipping sstable 989 | 21:57:16,845 | 10.10.100.5 | 15146 Partition index with 0 entries found for sstable 937 | 21:57:16,845 | 10.10.100.5 | 15565 Seeking to partition indexed section in data file | 21:57:16,845 | 10.10.100.5 | 15581 Partition index with 7158 entries found for sstable 884 | 21:57:16,898 | 10.10.100.5 | 68644 Seeking to partition indexed section in data file | 21:57:16,899 | 10.10.100.5 | 69014 Partition index with 20819 entries found for sstable 733 | 21:57:16,916 | 10.10.100.5 | 86121 Seeking to partition indexed section in data file | 21:57:16,916 | 10.10.100.5 | 86412 Skipped 1/6 non-slice-intersecting sstables, included 0 due to tombstones | 21:57:16,916 | 10.10.100.5 | 86494 Merging data from memtables and 3 sstables | 21:57:16,916 | 10.10.100.5 | 86522 Read 193311 live and 0 tombstoned cells | 21:57:24,552 | 10.10.100.5 | 7722425 Request complete | 21:57:29,074 | 10.10.100.5 | 12244832 Mohammed From: Alex Major [mailto:al3...@gmail.com] Sent: Wednesday, September 17, 2014 3:47 AM To: user@cassandra.apache.orgmailto:user@cassandra.apache.org Subject: Re: no change observed in read latency after switching from EBS to SSD storage When you say you moved from EBS to SSD, do you mean the EBS HDD drives to EBS SSD drives? Or instance SSD drives? The m3.large only comes with 32GB of instance based SSD storage. If you're using EBS SSD drives then network will still be the slowest thing so switching won't likely make much of a difference. On Wed, Sep 17, 2014 at 6:00 AM
Re: no change observed in read latency after switching from EBS to SSD storage
It is possible this is CPU bound. In 2.1 we have optimised the comparison of clustering columns (CASSANDRA-5417 https://issues.apache.org/jira/browse/CASSANDRA-5417), but in 2.0 it is quite expensive. So for a large row with several million comparisons to perform (to merge, filter, etc.) it could be a significant proportion of the cost. Note that these costs for a given query are all bound by a single core, there is no parallelism, since the assumption is we are serving more queries at once than there are cores (in general Cassandra is not designed to serve workloads consisting of single large queries, at least not yet) On Thu, Sep 18, 2014 at 7:29 AM, Mohammed Guller moham...@glassbeam.com wrote: Chris, I agree that reading 250k row is a bit excessive and that breaking up the partition would help reduce the query time. That part is well understood. The part that we can’t figure out is why read time did not change when we switched from a slow Network Attached Storage (AWS EBS) to local SSD. One possibility is that the read is not bound by disk i/o, but it is not cpu or memory bound either. So where is it spending all that time? Another possibility is that even though it is returning only 193311 cells, C* reads the entire partition, which may have a lot more cells. But even in that case reading from a local SSD should have been a lot faster than reading from non-provisioned EBS. Mohammed *From:* Chris Lohfink [mailto:clohf...@blackbirdit.com] *Sent:* Wednesday, September 17, 2014 7:17 PM *To:* user@cassandra.apache.org *Subject:* Re: no change observed in read latency after switching from EBS to SSD storage Read 193311 live and 0 tombstoned cells is your killer. returning 250k rows is a bit excessive, you should really page this in smaller chunks, what client are you using to access the data? This partition (a, b, c, d, e, f) may be too large as well (can check partition max size from output of *nodetool cfstats*), may be worth including g to break it up more - but I dont know enough about your data model. --- Chris Lohfink On Sep 17, 2014, at 4:53 PM, Mohammed Guller moham...@glassbeam.com wrote: Thank you all for your responses. Alex – Instance (ephemeral) SSD Ben – the query reads data from just one partition. If disk i/o is the bottleneck, then in theory, if reading from EBS takes 10 seconds, then it should take lot less when reading the same amount of data from local SSD. My question is not about why it is taking 10 seconds, but why is the read time same for both EBS (network attached storage) and local SSD? Tony – if the data was cached in memory, then a read should not take 10 seconds just for 20MB data Rob – Here is the schema, query, and trace. I masked the actual column names to protect the innocents J create table dummy( a varchar, b varchar, c varchar, d varchar, e varchar, f varchar, g varchar, h timestamp, i int, non_key1 varchar, ... non_keyN varchar, PRIMARY KEY ((a, b, c, d, e, f), g, h, i) ) WITH CLUSTERING ORDER BY (g ASC, h DESC, i ASC) SELECT h, non_key100, non_key200 FROM dummy WHERE a='' AND b='bb' AND c='ccc' AND d='dd' AND e='' AND f='ff' AND g='g'AND h ='2014-09-10T00:00:00' AND h='2014-09-10T23:40:41'; The above query returns around 250,000 CQL rows. cqlsh trace: activity | timestamp| source | source_elapsed - execute_cql3_query | 21:57:16,830 | 10.10.100.5 | 0 Parsing query; | 21:57:16,830 | 10.10.100.5 |673 Preparing statement | 21:57:16,831 | 10.10.100.5 | 1602 Executing single-partition query on event | 21:57:16,845 | 10.10.100.5 | 14871 Acquiring sstable references | 21:57:16,845 | 10.10.100.5 | 14896 Merging memtable tombstones | 21:57:16,845 | 10.10.100.5 | 14954 Bloom filter allows skipping sstable 1049 | 21:57:16,845 | 10.10.100.5 | 15090 Bloom filter allows skipping sstable 989 | 21:57:16,845 | 10.10.100.5 | 15146 Partition index with 0 entries found for sstable 937 | 21:57:16,845 | 10.10.100.5 | 15565 Seeking to partition indexed section in data file | 21:57:16,845 | 10.10.100.5 | 15581 Partition index with 7158 entries found for sstable 884 | 21:57:16,898 | 10.10.100.5 | 68644 Seeking to partition indexed section in data file | 21:57:16,899 | 10.10.100.5 | 69014 Partition index with 20819 entries found for sstable 733 | 21:57:16,916 | 10.10.100.5 | 86121 Seeking to partition indexed section in data file | 21:57:16,916 | 10.10.100.5 | 86412 Skipped 1/6 non-slice-intersecting sstables, included 0 due to tombstones | 21:57:16,916 | 10.10.100.5 | 86494 Merging data from
RE: no change observed in read latency after switching from EBS to SSD storage
Benedict, That makes perfect sense. Even though the node has multiple cores, I do see that only one core is pegged at 100%. Interestingly, after I switched to 2.1, cqlsh trace now shows that the same query takes only 600ms. However, cqlsh still waits for almost 20-30 seconds before it starts showing the result. I noticed similar latency when I ran the query from our app, which uses the Astyanax driver. So I thought perhaps there is a bug in the cqlsh code that tracks the statistics and the reported numbers are incorrect. But, I guess the numbers shown by cqlsh trace is correct, but the bottleneck is somewhere else now. In other words, the read operation itself is much faster in 2.1, but something else delays the response back to the client. Mohammed From: Benedict Elliott Smith [mailto:belliottsm...@datastax.com] Sent: Thursday, September 18, 2014 2:15 AM To: user@cassandra.apache.org Cc: Chris Lohfink Subject: Re: no change observed in read latency after switching from EBS to SSD storage It is possible this is CPU bound. In 2.1 we have optimised the comparison of clustering columns (CASSANDRA-5417https://issues.apache.org/jira/browse/CASSANDRA-5417), but in 2.0 it is quite expensive. So for a large row with several million comparisons to perform (to merge, filter, etc.) it could be a significant proportion of the cost. Note that these costs for a given query are all bound by a single core, there is no parallelism, since the assumption is we are serving more queries at once than there are cores (in general Cassandra is not designed to serve workloads consisting of single large queries, at least not yet) On Thu, Sep 18, 2014 at 7:29 AM, Mohammed Guller moham...@glassbeam.commailto:moham...@glassbeam.com wrote: Chris, I agree that reading 250k row is a bit excessive and that breaking up the partition would help reduce the query time. That part is well understood. The part that we can’t figure out is why read time did not change when we switched from a slow Network Attached Storage (AWS EBS) to local SSD. One possibility is that the read is not bound by disk i/o, but it is not cpu or memory bound either. So where is it spending all that time? Another possibility is that even though it is returning only 193311 cells, C* reads the entire partition, which may have a lot more cells. But even in that case reading from a local SSD should have been a lot faster than reading from non-provisioned EBS. Mohammed From: Chris Lohfink [mailto:clohf...@blackbirdit.commailto:clohf...@blackbirdit.com] Sent: Wednesday, September 17, 2014 7:17 PM To: user@cassandra.apache.orgmailto:user@cassandra.apache.org Subject: Re: no change observed in read latency after switching from EBS to SSD storage Read 193311 live and 0 tombstoned cells is your killer. returning 250k rows is a bit excessive, you should really page this in smaller chunks, what client are you using to access the data? This partition (a, b, c, d, e, f) may be too large as well (can check partition max size from output of nodetool cfstats), may be worth including g to break it up more - but I dont know enough about your data model. --- Chris Lohfink On Sep 17, 2014, at 4:53 PM, Mohammed Guller moham...@glassbeam.commailto:moham...@glassbeam.com wrote: Thank you all for your responses. Alex – Instance (ephemeral) SSD Ben – the query reads data from just one partition. If disk i/o is the bottleneck, then in theory, if reading from EBS takes 10 seconds, then it should take lot less when reading the same amount of data from local SSD. My question is not about why it is taking 10 seconds, but why is the read time same for both EBS (network attached storage) and local SSD? Tony – if the data was cached in memory, then a read should not take 10 seconds just for 20MB data Rob – Here is the schema, query, and trace. I masked the actual column names to protect the innocents ☺ create table dummy( a varchar, b varchar, c varchar, d varchar, e varchar, f varchar, g varchar, h timestamp, i int, non_key1 varchar, ... non_keyN varchar, PRIMARY KEY ((a, b, c, d, e, f), g, h, i) ) WITH CLUSTERING ORDER BY (g ASC, h DESC, i ASC) SELECT h, non_key100, non_key200 FROM dummy WHERE a='' AND b='bb' AND c='ccc' AND d='dd' AND e='' AND f='ff' AND g='g'AND h ='2014-09-10T00:00:00' AND h='2014-09-10T23:40:41'; The above query returns around 250,000 CQL rows. cqlsh trace: activity | timestamp| source | source_elapsed - execute_cql3_query | 21:57:16,830 | 10.10.100.5 | 0 Parsing query; | 21:57:16,830 | 10.10.100.5 |673 Preparing statement | 21:57:16,831 | 10.10.100.5 | 1602 Executing single-partition query on event | 21:57:16,845 | 10.10.100.5 | 14871 Acquiring sstable references | 21:57:16,845 | 10.10.100.5
Re: no change observed in read latency after switching from EBS to SSD storage
When you say you moved from EBS to SSD, do you mean the EBS HDD drives to EBS SSD drives? Or instance SSD drives? The m3.large only comes with 32GB of instance based SSD storage. If you're using EBS SSD drives then network will still be the slowest thing so switching won't likely make much of a difference. On Wed, Sep 17, 2014 at 6:00 AM, Mohammed Guller moham...@glassbeam.com wrote: Rob, The 10 seconds latency that I gave earlier is from CQL tracing. Almost 5 seconds out of that was taken up by the “merge memtable and sstables” step. The remaining 5 seconds are from “read live and tombstoned cells.” I too first thought that maybe disk is not the bottleneck and Cassandra is serving everything from cache, but in that case, it should not take 10 seconds for reading just 20MB data. Also, I narrowed down the query to limit it to a single partition read and I ran the query in cqlsh running on the same node. I turned on tracing, which shows that all the steps got executed on the same node. htop shows that CPU and memory are not the bottlenecks. Network should not come into play since the cqlsh is running on the same node. Is there any performance tuning parameter in the cassandra.yaml file for large reads? Mohammed *From:* Robert Coli [mailto:rc...@eventbrite.com] *Sent:* Tuesday, September 16, 2014 5:42 PM *To:* user@cassandra.apache.org *Subject:* Re: no change observed in read latency after switching from EBS to SSD storage On Tue, Sep 16, 2014 at 5:35 PM, Mohammed Guller moham...@glassbeam.com wrote: Does anyone have insight as to why we don't see any performance impact on the reads going from EBS to SSD? What does it say when you enable tracing on this CQL query? 10 seconds is a really long time to access anything in Cassandra. There is, generally speaking, a reason why the default timeouts are lower than this. My conjecture is that the data in question was previously being served from the page cache and is now being served from SSD. You have, in switching from EBS-plus-page-cache to SSD successfully proved that SSD and RAM are both very fast. There is also a strong suggestion that whatever access pattern you are using is not bounded by disk performance. =Rob
Re: no change observed in read latency after switching from EBS to SSD storage
On Tue, Sep 16, 2014 at 10:00 PM, Mohammed Guller moham...@glassbeam.com wrote: The 10 seconds latency that I gave earlier is from CQL tracing. Almost 5 seconds out of that was taken up by the “merge memtable and sstables” step. The remaining 5 seconds are from “read live and tombstoned cells.” Could you paste the query, the schema, and the trace? Your summary is not likely to be as usefully diagnostic as these.. :) =Rob
RE: no change observed in read latency after switching from EBS to SSD storage
Thank you all for your responses. Alex – Instance (ephemeral) SSD Ben – the query reads data from just one partition. If disk i/o is the bottleneck, then in theory, if reading from EBS takes 10 seconds, then it should take lot less when reading the same amount of data from local SSD. My question is not about why it is taking 10 seconds, but why is the read time same for both EBS (network attached storage) and local SSD? Tony – if the data was cached in memory, then a read should not take 10 seconds just for 20MB data Rob – Here is the schema, query, and trace. I masked the actual column names to protect the innocents ☺ create table dummy( a varchar, b varchar, c varchar, d varchar, e varchar, f varchar, g varchar, h timestamp, i int, non_key1 varchar, ... non_keyN varchar, PRIMARY KEY ((a, b, c, d, e, f), g, h, i) ) WITH CLUSTERING ORDER BY (g ASC, h DESC, i ASC) SELECT h, non_key100, non_key200 FROM dummy WHERE a='' AND b='bb' AND c='ccc' AND d='dd' AND e='' AND f='ff' AND g='g'AND h ='2014-09-10T00:00:00' AND h='2014-09-10T23:40:41'; The above query returns around 250,000 CQL rows. cqlsh trace: activity | timestamp| source | source_elapsed - execute_cql3_query | 21:57:16,830 | 10.10.100.5 | 0 Parsing query; | 21:57:16,830 | 10.10.100.5 |673 Preparing statement | 21:57:16,831 | 10.10.100.5 | 1602 Executing single-partition query on event | 21:57:16,845 | 10.10.100.5 | 14871 Acquiring sstable references | 21:57:16,845 | 10.10.100.5 | 14896 Merging memtable tombstones | 21:57:16,845 | 10.10.100.5 | 14954 Bloom filter allows skipping sstable 1049 | 21:57:16,845 | 10.10.100.5 | 15090 Bloom filter allows skipping sstable 989 | 21:57:16,845 | 10.10.100.5 | 15146 Partition index with 0 entries found for sstable 937 | 21:57:16,845 | 10.10.100.5 | 15565 Seeking to partition indexed section in data file | 21:57:16,845 | 10.10.100.5 | 15581 Partition index with 7158 entries found for sstable 884 | 21:57:16,898 | 10.10.100.5 | 68644 Seeking to partition indexed section in data file | 21:57:16,899 | 10.10.100.5 | 69014 Partition index with 20819 entries found for sstable 733 | 21:57:16,916 | 10.10.100.5 | 86121 Seeking to partition indexed section in data file | 21:57:16,916 | 10.10.100.5 | 86412 Skipped 1/6 non-slice-intersecting sstables, included 0 due to tombstones | 21:57:16,916 | 10.10.100.5 | 86494 Merging data from memtables and 3 sstables | 21:57:16,916 | 10.10.100.5 | 86522 Read 193311 live and 0 tombstoned cells | 21:57:24,552 | 10.10.100.5 | 7722425 Request complete | 21:57:29,074 | 10.10.100.5 | 12244832 Mohammed From: Alex Major [mailto:al3...@gmail.com] Sent: Wednesday, September 17, 2014 3:47 AM To: user@cassandra.apache.org Subject: Re: no change observed in read latency after switching from EBS to SSD storage When you say you moved from EBS to SSD, do you mean the EBS HDD drives to EBS SSD drives? Or instance SSD drives? The m3.large only comes with 32GB of instance based SSD storage. If you're using EBS SSD drives then network will still be the slowest thing so switching won't likely make much of a difference. On Wed, Sep 17, 2014 at 6:00 AM, Mohammed Guller moham...@glassbeam.commailto:moham...@glassbeam.com wrote: Rob, The 10 seconds latency that I gave earlier is from CQL tracing. Almost 5 seconds out of that was taken up by the “merge memtable and sstables” step. The remaining 5 seconds are from “read live and tombstoned cells.” I too first thought that maybe disk is not the bottleneck and Cassandra is serving everything from cache, but in that case, it should not take 10 seconds for reading just 20MB data. Also, I narrowed down the query to limit it to a single partition read and I ran the query in cqlsh running on the same node. I turned on tracing, which shows that all the steps got executed on the same node. htop shows that CPU and memory are not the bottlenecks. Network should not come into play since the cqlsh is running on the same node. Is there any performance tuning parameter in the cassandra.yaml file for large reads? Mohammed From: Robert Coli [mailto:rc...@eventbrite.commailto:rc...@eventbrite.com] Sent: Tuesday, September 16, 2014 5:42 PM To: user@cassandra.apache.orgmailto:user@cassandra.apache.org Subject: Re: no change observed in read latency after switching from EBS to SSD storage On Tue, Sep 16, 2014 at 5:35 PM, Mohammed Guller moham...@glassbeam.commailto:moham...@glassbeam.com wrote: Does anyone have insight as to why we don't see any performance impact on the reads going from EBS to SSD? What does it say when you enable tracing on this CQL query? 10 seconds is a really long
Re: no change observed in read latency after switching from EBS to SSD storage
Read 193311 live and 0 tombstoned cells is your killer. returning 250k rows is a bit excessive, you should really page this in smaller chunks, what client are you using to access the data? This partition (a, b, c, d, e, f) may be too large as well (can check partition max size from output of nodetool cfstats), may be worth including g to break it up more - but I dont know enough about your data model. --- Chris Lohfink On Sep 17, 2014, at 4:53 PM, Mohammed Guller moham...@glassbeam.com wrote: Thank you all for your responses. Alex – Instance (ephemeral) SSD Ben – the query reads data from just one partition. If disk i/o is the bottleneck, then in theory, if reading from EBS takes 10 seconds, then it should take lot less when reading the same amount of data from local SSD. My question is not about why it is taking 10 seconds, but why is the read time same for both EBS (network attached storage) and local SSD? Tony – if the data was cached in memory, then a read should not take 10 seconds just for 20MB data Rob – Here is the schema, query, and trace. I masked the actual column names to protect the innocents J create table dummy( a varchar, b varchar, c varchar, d varchar, e varchar, f varchar, g varchar, h timestamp, i int, non_key1 varchar, ... non_keyN varchar, PRIMARY KEY ((a, b, c, d, e, f), g, h, i) ) WITH CLUSTERING ORDER BY (g ASC, h DESC, i ASC) SELECT h, non_key100, non_key200 FROM dummy WHERE a='' AND b='bb' AND c='ccc' AND d='dd' AND e='' AND f='ff' AND g='g'AND h ='2014-09-10T00:00:00' AND h='2014-09-10T23:40:41'; The above query returns around 250,000 CQL rows. cqlsh trace: activity | timestamp| source | source_elapsed - execute_cql3_query | 21:57:16,830 | 10.10.100.5 | 0 Parsing query; | 21:57:16,830 | 10.10.100.5 |673 Preparing statement | 21:57:16,831 | 10.10.100.5 | 1602 Executing single-partition query on event | 21:57:16,845 | 10.10.100.5 | 14871 Acquiring sstable references | 21:57:16,845 | 10.10.100.5 | 14896 Merging memtable tombstones | 21:57:16,845 | 10.10.100.5 | 14954 Bloom filter allows skipping sstable 1049 | 21:57:16,845 | 10.10.100.5 | 15090 Bloom filter allows skipping sstable 989 | 21:57:16,845 | 10.10.100.5 | 15146 Partition index with 0 entries found for sstable 937 | 21:57:16,845 | 10.10.100.5 | 15565 Seeking to partition indexed section in data file | 21:57:16,845 | 10.10.100.5 | 15581 Partition index with 7158 entries found for sstable 884 | 21:57:16,898 | 10.10.100.5 | 68644 Seeking to partition indexed section in data file | 21:57:16,899 | 10.10.100.5 | 69014 Partition index with 20819 entries found for sstable 733 | 21:57:16,916 | 10.10.100.5 | 86121 Seeking to partition indexed section in data file | 21:57:16,916 | 10.10.100.5 | 86412 Skipped 1/6 non-slice-intersecting sstables, included 0 due to tombstones | 21:57:16,916 | 10.10.100.5 | 86494 Merging data from memtables and 3 sstables | 21:57:16,916 | 10.10.100.5 | 86522 Read 193311 live and 0 tombstoned cells | 21:57:24,552 | 10.10.100.5 | 7722425 Request complete | 21:57:29,074 | 10.10.100.5 | 12244832 Mohammed From: Alex Major [mailto:al3...@gmail.com] Sent: Wednesday, September 17, 2014 3:47 AM To: user@cassandra.apache.org Subject: Re: no change observed in read latency after switching from EBS to SSD storage When you say you moved from EBS to SSD, do you mean the EBS HDD drives to EBS SSD drives? Or instance SSD drives? The m3.large only comes with 32GB of instance based SSD storage. If you're using EBS SSD drives then network will still be the slowest thing so switching won't likely make much of a difference. On Wed, Sep 17, 2014 at 6:00 AM, Mohammed Guller moham...@glassbeam.com wrote: Rob, The 10 seconds latency that I gave earlier is from CQL tracing. Almost 5 seconds out of that was taken up by the “merge memtable and sstables” step. The remaining 5 seconds are from “read live and tombstoned cells.” I too first thought that maybe disk is not the bottleneck and Cassandra is serving everything from cache, but in that case, it should not take 10 seconds for reading just 20MB data. Also, I narrowed down the query to limit it to a single partition read and I ran the query in cqlsh running on the same node. I turned on tracing, which shows that all the steps got executed on the same node. htop shows that CPU and memory are not the bottlenecks. Network should not come into play since the cqlsh is running on the same node. Is there any performance tuning parameter in the cassandra.yaml
no change observed in read latency after switching from EBS to SSD storage
Hi - We are running Cassandra 2.0.5 on AWS on m3.large instances. These instances were using EBS for storage (I know it is not recommended). We replaced the EBS storage with SSDs. However, we didn't see any change in read latency. A query that took 10 seconds when data was stored on EBS still takes 10 seconds even after we moved the data directory to SSD. It is a large query returning 200,000 CQL rows from a single partition. We are reading 3 columns from each row and the combined data in these three columns for each row is around 100 bytes. In other words, the raw data returned by the query is approximately 20MB. I was expecting at least 5-10 times reduction in read latency going from EBS to SSD, so I am puzzled why we are not seeing any change in performance. Does anyone have insight as to why we don't see any performance impact on the reads going from EBS to SSD? Thanks, Mohammed
Re: no change observed in read latency after switching from EBS to SSD storage
On Tue, Sep 16, 2014 at 5:35 PM, Mohammed Guller moham...@glassbeam.com wrote: Does anyone have insight as to why we don't see any performance impact on the reads going from EBS to SSD? What does it say when you enable tracing on this CQL query? 10 seconds is a really long time to access anything in Cassandra. There is, generally speaking, a reason why the default timeouts are lower than this. My conjecture is that the data in question was previously being served from the page cache and is now being served from SSD. You have, in switching from EBS-plus-page-cache to SSD successfully proved that SSD and RAM are both very fast. There is also a strong suggestion that whatever access pattern you are using is not bounded by disk performance. =Rob
Re: no change observed in read latency after switching from EBS to SSD storage
To expand on what Robert said, Cassandra is a log-structured database: - writes are append operations, so both correctly configured disk volumes and SSD are fast at that - reads could be helped by SSD if they're not in cache (ie. on disk) - but compaction is definitely helped by SSD with large data loads (compaction is the trade-off for fast writes) Thanks, James Briggs. -- Cassandra/MySQL DBA. Available in San Jose area or remote. Mailbox dimensions: 10x12x14 From: Robert Coli rc...@eventbrite.com To: user@cassandra.apache.org user@cassandra.apache.org Sent: Tuesday, September 16, 2014 5:42 PM Subject: Re: no change observed in read latency after switching from EBS to SSD storage On Tue, Sep 16, 2014 at 5:35 PM, Mohammed Guller moham...@glassbeam.com wrote: Does anyone have insight as to why we don't see any performance impact on the reads going from EBS to SSD? What does it say when you enable tracing on this CQL query? 10 seconds is a really long time to access anything in Cassandra. There is, generally speaking, a reason why the default timeouts are lower than this. My conjecture is that the data in question was previously being served from the page cache and is now being served from SSD. You have, in switching from EBS-plus-page-cache to SSD successfully proved that SSD and RAM are both very fast. There is also a strong suggestion that whatever access pattern you are using is not bounded by disk performance. =Rob
Re: no change observed in read latency after switching from EBS to SSD storage
Mohammed, to add to previous answers, EBS is network attached, with SSD or without it , you access your disk via network constrained by network bandwidth and latency, if you really need to improve IO performance try switching to ephemeral storage (also called instance storage) which is physically attached to EC2 instance, and is as good as native disk IO goes. On Tue, Sep 16, 2014 at 11:39 PM, James Briggs james.bri...@yahoo.com wrote: To expand on what Robert said, Cassandra is a log-structured database: - writes are append operations, so both correctly configured disk volumes and SSD are fast at that - reads could be helped by SSD if they're not in cache (ie. on disk) - but compaction is definitely helped by SSD with large data loads (compaction is the trade-off for fast writes) Thanks, James Briggs. -- Cassandra/MySQL DBA. Available in San Jose area or remote. Mailbox dimensions: 10x12x14 -- *From:* Robert Coli rc...@eventbrite.com *To:* user@cassandra.apache.org user@cassandra.apache.org *Sent:* Tuesday, September 16, 2014 5:42 PM *Subject:* Re: no change observed in read latency after switching from EBS to SSD storage On Tue, Sep 16, 2014 at 5:35 PM, Mohammed Guller moham...@glassbeam.com wrote: Does anyone have insight as to why we don't see any performance impact on the reads going from EBS to SSD? What does it say when you enable tracing on this CQL query? 10 seconds is a really long time to access anything in Cassandra. There is, generally speaking, a reason why the default timeouts are lower than this. My conjecture is that the data in question was previously being served from the page cache and is now being served from SSD. You have, in switching from EBS-plus-page-cache to SSD successfully proved that SSD and RAM are both very fast. There is also a strong suggestion that whatever access pattern you are using is not bounded by disk performance. =Rob
Re: no change observed in read latency after switching from EBS to SSD storage
If you cached your tables or the database you may not see any difference at all. Regards, -Tony On Tuesday, September 16, 2014 6:36 PM, Mohammed Guller moham...@glassbeam.com wrote: Hi - We are running Cassandra 2.0.5 on AWS on m3.large instances. These instances were using EBS for storage (I know it is not recommended). We replaced the EBS storage with SSDs. However, we didn't see any change in read latency. A query that took 10 seconds when data was stored on EBS still takes 10 seconds even after we moved the data directory to SSD. It is a large query returning 200,000 CQL rows from a single partition. We are reading 3 columns from each row and the combined data in these three columns for each row is around 100 bytes. In other words, the raw data returned by the query is approximately 20MB. I was expecting at least 5-10 times reduction in read latency going from EBS to SSD, so I am puzzled why we are not seeing any change in performance. Does anyone have insight as to why we don't see any performance impact on the reads going from EBS to SSD? Thanks, Mohammed
Re: no change observed in read latency after switching from EBS to SSD storage
EBS vs local SSD in terms of latency you are using ms as your unit of measurement. If your query runs for 10s you will not notice anything. What is a few less ms for the life of a 10 second query. To reiterate what Rob said. The query is probably slow because of your use case / data model, not the underlying disk. On 17 September 2014 14:21, Tony Anecito adanec...@yahoo.com wrote: If you cached your tables or the database you may not see any difference at all. Regards, -Tony On Tuesday, September 16, 2014 6:36 PM, Mohammed Guller moham...@glassbeam.com wrote: Hi - We are running Cassandra 2.0.5 on AWS on m3.large instances. These instances were using EBS for storage (I know it is not recommended). We replaced the EBS storage with SSDs. However, we didn't see any change in read latency. A query that took 10 seconds when data was stored on EBS still takes 10 seconds even after we moved the data directory to SSD. It is a large query returning 200,000 CQL rows from a single partition. We are reading 3 columns from each row and the combined data in these three columns for each row is around 100 bytes. In other words, the raw data returned by the query is approximately 20MB. I was expecting at least 5-10 times reduction in read latency going from EBS to SSD, so I am puzzled why we are not seeing any change in performance. Does anyone have insight as to why we don't see any performance impact on the reads going from EBS to SSD? Thanks, Mohammed -- Ben Bromhead Instaclustr | www.instaclustr.com | @instaclustr http://twitter.com/instaclustr | +61 415 936 359
RE: no change observed in read latency after switching from EBS to SSD storage
Rob, The 10 seconds latency that I gave earlier is from CQL tracing. Almost 5 seconds out of that was taken up by the “merge memtable and sstables” step. The remaining 5 seconds are from “read live and tombstoned cells.” I too first thought that maybe disk is not the bottleneck and Cassandra is serving everything from cache, but in that case, it should not take 10 seconds for reading just 20MB data. Also, I narrowed down the query to limit it to a single partition read and I ran the query in cqlsh running on the same node. I turned on tracing, which shows that all the steps got executed on the same node. htop shows that CPU and memory are not the bottlenecks. Network should not come into play since the cqlsh is running on the same node. Is there any performance tuning parameter in the cassandra.yaml file for large reads? Mohammed From: Robert Coli [mailto:rc...@eventbrite.com] Sent: Tuesday, September 16, 2014 5:42 PM To: user@cassandra.apache.org Subject: Re: no change observed in read latency after switching from EBS to SSD storage On Tue, Sep 16, 2014 at 5:35 PM, Mohammed Guller moham...@glassbeam.commailto:moham...@glassbeam.com wrote: Does anyone have insight as to why we don't see any performance impact on the reads going from EBS to SSD? What does it say when you enable tracing on this CQL query? 10 seconds is a really long time to access anything in Cassandra. There is, generally speaking, a reason why the default timeouts are lower than this. My conjecture is that the data in question was previously being served from the page cache and is now being served from SSD. You have, in switching from EBS-plus-page-cache to SSD successfully proved that SSD and RAM are both very fast. There is also a strong suggestion that whatever access pattern you are using is not bounded by disk performance. =Rob
