Re: 10K vs 15k rpm for analytics

On Mar 8, 2010, at 11:00 PM, Greg Smith wrote:

> Scott Carey wrote:
>> For high sequential throughput, nothing is as optimized as XFS on Linux yet. It has weaknesses elsewhere however.
>>
>
> I'm curious what you feel those weaknesses are. The recent addition of
> XFS back into a more mainstream position in the RHEL kernel as of their
> 5.4 update greatly expands where I can use it now, have been heavily
> revisiting it since that release. I've already noted how well it does
> on sequential read/write tasks relative to ext3, and it looks like the
> main downsides I used to worry about with it (mainly crash recovery
> issues) were also squashed in recent years.
>

My somewhat negative experiences have been:

* Metadata operations are a bit slow, this manifests itself mostly with lots of small files being updated or deleted.
* Improper use of the file system or hardware configuration will likely break worse (ext3 'ordered' mode makes poorly written apps safer).
* At least with CentOS 5.3 and thier xfs version (non-Redhat, CentOS extras) sparse random writes could almost hang a file system. They were VERY slow. I have not tested since.

None of the above affect Postgres.

I'm also not sure how up to date RedHat's xfs version is -- there have been enhancements to xfs in the kernel mainline regularly for a long time.

In non-postgres contexts, I've grown to appreciate some other qualities: Unlike ext2/3, I can have more than 32K directories in another directory -- XFS will do millions, though it will slow down at least it doesn't just throw an error to the application. And although XFS is slow to delete lots of small things, it can delete large files much faster -- I deal with lots of large files and it is comical to see ext3 take a minute to delete a 30GB file while XFS does it almost instantly.

I have been happy with XFS for Postgres data directories, and ext2 for a dedicated xlog partition. Although I have not risked the online defragmentation on a live DB, I have defragmented a 8TB DB during maintenance and seen the performance improve.

> --
> Greg Smith 2ndQuadrant US Baltimore, MD
> PostgreSQL Training, Services and Support
> greg(at)2ndQuadrant(dot)com www.2ndQuadrant.us
>