Re: CoW with webserver databases: innodb_file_per_table and dedicated tables for blobs?

[Ingvar Bogdahn <ingvar.bogdahn@googlemail.com>]

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.
>