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https://issues.apache.org/jira/browse/CASSANDRA-1625?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12971877#action_12971877
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Peter Schuller edited comment on CASSANDRA-1625 at 12/15/10 5:55 PM:
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Synergy!
I just realized that if we combine Stu's suggestion with:
(1) CASSANDRA-1657 (in-memory cf)
(2) Just have the CF *be* the "cache" (I agree with Stu's problem with calling
it a cache) rather than being used only on start-up to populate the row cache
(3) Ongoing improvements to compaction (whether it's fadvise/mincore or others)
that eliminates the need to keep both the old and the new sstables in memory
We now have, at least when it comes to eliminating disk I/O, a in-memory cache
that is:
(1) Memory efficient due to being backed by tightly packed sstables as usual
(subject to compaction thresholds).
(2) Not made up of objects in the heap, so no GC pressure trade-off (except for
tracking recenticity information).
(3) Under much more explicit control in terms of size; while it would not come
for free, being able to say "have the cache be N MB large" would not be
unrealistic.
(4) Extremely efficiently instantly available all hot and cozy on start-up
regardless of total data set size; no seek bound operations in sight.
Theoretically as efficiently as an mmap() (edited: accidentally put mlock())
with MAP_POPULATE, depending on CASSANDRA-1657
I say "at least when it comes to eliminating disk I/O" because presumably
reading from such a CF is more expensive than using the currently existing row
cache.
The two approaches are not mutually exclusive though and could even be used
together for the same CF if a situation should so demand.
The lack of (3) means the memory efficiency is lessened, but in other respects
the positive effects should remain.
But wait, there is more, as they say: If we ignore CASSANDRA-1657 for a moment,
and instead replace it by some logic to keep a given CF on a different
path/mountpoint, Cassandra would now have the capability of keeping the hot set
persistent on faster storage. In other words, this is direct support for
utilizing a fast SSD in a system which otherwise has lots of bulk storage -
without relying on something OS specific like ZFS L2ARC, and with, it seems,
minimal (for the gains) implementation effort.
This has me all excited, so I hope no one points out some obvious flaw in this
plan :)
was (Author: scode):
Synergy!
I just realized that if we combine Stu's suggestion with:
(1) CASSANDRA-1657 (in-memory cf)
(2) Just have the CF *be* the "cache" (I agree with Stu's problem with calling
it a cache) rather than being used only on start-up to populate the row cache
(3) Ongoing improvements to compaction (whether it's fadvise/mincore or others)
that eliminates the need to keep both the old and the new sstables in memory
We now have, at least when it comes to eliminating disk I/O, a in-memory cache
that is:
(1) Memory efficient due to being backed by tightly packed sstables as usual
(subject to compaction thresholds).
(2) Not made up of objects in the heap, so no GC pressure trade-off (except for
tracking recenticity information).
(3) Under much more explicit control in terms of size; while it would not come
for free, being able to say "have the cache be N MB large" would not be
unrealistic.
(4) Extremely efficiently instantly available all hot and cozy on start-up
regardless of total data set size; no seek bound operations in sight.
Theoretically as efficiently as an mlock() with MAP_POPULATE, depending on
CASSANDRA-1657
I say "at least when it comes to eliminating disk I/O" because presumably
reading from such a CF is more expensive than using the currently existing row
cache.
The two approaches are not mutually exclusive though and could even be used
together for the same CF if a situation should so demand.
The lack of (3) means the memory efficiency is lessened, but in other respects
the positive effects should remain.
But wait, there is more, as they say: If we ignore CASSANDRA-1657 for a moment,
and instead replace it by some logic to keep a given CF on a different
path/mountpoint, Cassandra would now have the capability of keeping the hot set
persistent on faster storage. In other words, this is direct support for
utilizing a fast SSD in a system which otherwise has lots of bulk storage -
without relying on something OS specific like ZFS L2ARC, and with, it seems,
minimal (for the gains) implementation effort.
This has me all excited, so I hope no one points out some obvious flaw in this
plan :)
> make the row cache continuously durable
> ---------------------------------------
>
> Key: CASSANDRA-1625
> URL: https://issues.apache.org/jira/browse/CASSANDRA-1625
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Reporter: Peter Schuller
> Priority: Minor
>
> I was looking into how the row cache worked today and realized only row keys
> were saved and later pre-populated on start-up.
> On the premise that row caches are typically used for small rows of which
> there may be many, this is highly likely to be seek bound on large data sets
> during pre-population.
> The pre-population could be made faster by increasing I/O queue depth (by
> concurrency or by libaio as in 1576), but especially on large data sets the
> performance would be nowhere near what could be achieved if a reasonably
> sized file containing the actual rows were to be read in a sequential fashion
> on start.
> On the one hand, Cassandra's design means that this should be possible to do
> efficiently much easier than in some other cases, but on the other hand it is
> still not entirely trivial.
> The key problem with maintaining a continuously durable cache is that one
> must never read stale data on start-up. Stale could mean either data that was
> later deleted, or an old version of data that was updated.
> In the case of Cassandra, this means that any cache restored on start-up must
> be up-to-date with whatever position in the commit log that commit log
> recovery will start at. (Because the row cache is for an entire row, we can't
> couple updating of an on-disk row cache with memtable flushes.)
> I can see two main approaches:
> (a) Periodically dump the entire row cache, deferring commit log eviction in
> synchronization with said dumping.
> (b) Keep a change log of sorts, similar to the commit log but filtered to
> only contain data written to the commit log that affects keys that were in
> the row cache at the time. Eviction of commit logs or updating positional
> markers that affect the point of commit log recovery start, would imply
> fsync():ing this change log. An incremental traversal, or alternatively a
> periodic full dump, would have to be used to ensure that old row change log
> segments can be evicted without loss of cache warmness.
> I like (b), but it is also the introduction of significant complexity (and
> potential write path overhead) for the purpose of the row cache. In the worst
> case where hotly read data is also hotly written, the overhead could be
> particularly significant.
> I am not convinced whether this is a good idea for Cassandra, but I have a
> use-case where a similar cache might have to be written in the application to
> achieve the desired effect (pre-population being too slow for a sufficiently
> large row cache). But there are reasons why, in an ideal world, having such a
> continuously durable cache in Cassandra would be much better than something
> at the application level. The primary reason is that it does not interact
> poorly with consistency in the cluster, since the cache is node-local and
> appropriate measures would be taken to make it consistent locally on each
> node. I.e., it would be entirely transparent to the application.
> Thoughts? Like/dislike/too complex/not worth it?
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