Hi again,
Benchmarking over time seems a good idea, but what if I see that a
particular database does indeed degrade in performance? How can I then
selectively improve performance for that file, since disabling cow only
works for new empty files?
Is it correct that bundling small random writes into groups of writes
reduces fragmentation? If so, some form of write-caching should help?
I'm still investigating, but one solution might be:
1) identify which exact tables do have frequent writes
2) decrease the system-wide write-caching (vm.dirty_background_ratio and
vm.dirty_ratio) to lower levels, because this wastes lots of RAM by
indiscriminately caching writes of the whole system, and tends to causes
spikes where suddenly the entire cache gets written to disk and block
the system. Rather use that RAM selectively to cache only the critical
files.
4) create a software RAID-1 made up of a ramdisk and a mounted image,
using mdadm.
5) Setting up mdadm using rather large value for "write-behind="
6) put only those tables on that disk-backed ramdisk which do have
frequent writes.
What do you think?
Ingvar
Am 15.06.15 um 11:57 schrieb Hugo Mills:
On Mon, Jun 15, 2015 at 11:34:35AM +0200, Ingvar Bogdahn wrote:
Hello there,
I'm planing to use btrfs for a medium-sized webserver. It is
commonly recommended to set nodatacow for database files to avoid
performance degradation. However, apparently nodatacow disables some
of my main motivations of using btrfs : checksumming and (probably)
incremental backups with send/receive (please correct me if I'm
wrong on this). Also, the databases are among the most important
data on my webserver, so it is particularly there that I would like
those feature working.
My question is, are there strategies to avoid nodatacow of databases
that are suitable and safe in a production server?
I thought about the following:
- in mysql/mariadb: setting "innodb_file_per_table" should avoid
having few very big database files.
It's not so much about the overall size of the files, but about the
write patterns, so this probably won't be useful.
- in mysql/mariadb: adapting database schema to store blobs into
dedicated tables.
Probably not an issue -- each BLOB is (likely) to be written in a
single unit, which won't cause the fragmentation problems.
- btrfs: set autodefrag or some cron job to regularly defrag only
database fails to avoid performance degradation due to fragmentation
Autodefrag is a good idea, and I would suggest trying that first,
before anything else, to see if it gives you good enough performance
over time.
Running an explicit defrag will break any CoW copies you have (like
snapshots), causing them to take up additional space. For example,
start with a 10 GB subvolume. Snapshot it, and you will still only
have 10 GB of disk usage. Defrag one (or both) copies, and you'll
suddenly be using 20 GB.
- turn on compression on either btrfs or mariadb
Again, won't help. The issue is not the size of the data, it's the
write patterns: small random writes into the middle of existing files
will eventually cause those files to fragment, which causes lots of
seeks and short reads, which degrades performance.
Is this likely to give me ok-ish performance? What other
possibilities are there?
I would recommend benchmarking over time with your workloads, and
seeing how your performance degrades.
Hugo.
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