[mythtv] innodb vs myism preformance
Daniel Manjarres
danmanj at gmail.com
Fri Feb 18 03:06:32 UTC 2005
Yan-Fa Li wrote:
>Daniel Manjarres wrote:
>
>
>>One difference may be the filesystems we're using. I have ext3 on a
>>700GB lvm volume. The delays get really bad when deleting more than one
>>show on my system.
>>
>>
>>
>
>Interesting. I run the db on a separate file system. You're running
>everything on one file system ? Ext3 is kind of famous for slow deletes
>as I remember. My video is stored on xfs, my database on ext3. All my
>disks are RAID1 arrays with LVM overlays of smaller partitions. Have
>you tried moving the database to it's own file system separate from your
>video ?
>
>
My main unix filesystems are on 10,000 RPM scsi disks in raid 5. My tv
filesystem is slow, big IDE disks.
>From your description, it seems that mythtv is blocking mysql from
>doing file actions due to your filesystem choice. This is pretty relevant,
>because myisam is file based. Each table being 3 discrete files, data,
>index and meta data. Innodb is a binary blob and probably does a lot of
>caching in memory, deferring writes. I think this is why your
>performance goes up by switching db.
>
>
You know, you're prety smart, dude. But a 100% CPU spike tells me that
somebody (exactly 1 thread, since I have 2 cpus and one is mostly
idle)is spining on a resource lock, not waiting for disk acceess, and
since my GUI is ot responsive I am guessing it's the frontend that's
doing the waiting. Innodb is log structured, so that does reduce the
size of incremental update writes, but c'mon deleting a row shouldn't
take several seconds, no matter how busy the disk is, or else mysql
would die under low-moderate loads. Benchmarks are in kilo queries per
second, not seconds per query.
>I would try a couple of things:
>
> a) move the db files to a separate disk partition, or even as a
> test a different machine and use myisam.
>
>
Check. 10,000 RPM u160 scsi raid 5.
> b) if you can afford the downtime or loss of shows, try
> different filesystem types. I find xfs to be quite stable
> these days under 2.4.29 and I'm sure 2.6.10 is pretty good
> too.
>
>
Never going to happen. I am not losing my archives of The Daily Show.
Like I said I tried to relieve disk I/O pressure by repeatedly
truncating the file and sleeping for awhile instead of unlinking it, and
it made things WORSE, not better.
>Good luck,
>
>
Thanks for all the ideas, I'm sure somebody will get to the bottom of
this, eventually.
Dan Manjarres
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