ADSM-L

Re: Performance/striping/etc.

1998-09-04 14:41:04
Subject: Re: Performance/striping/etc.
From: Paul Hilchey <hilchey AT UCS.UBC DOT CA>
Date: Fri, 4 Sep 1998 11:41:04 -0700
On Sep 3,  6:52am, Bruce Elrick wrote:
> Given AIX 4.2.1 or higher on an RS/6000 with SSA disk, would the
> following be a reasonable disk allocation policy for performance and
> reliability?  I put my comments after each.  I welcome other comments as
> well.
>
> 1) Use ADSM mirroring for database and recovery log with mirrorwrite set
> to sequential for both
> - Yes, protection from corruption is the highest goal.
>
> 2) Raw logical volumes for all disk volumes (db, log, & stgpool)
> - JFS adds journaling but only for FS structure changes (inode
> allocation, directory structure changes) and thus prvides marginal
> protection.  The primary advantage of JFS is over other non-journaled
> FS's, protecting you from the need to do long fsck's after crashes.
> - Raw LV's waste some space ((PP size - 1MB) for db and log, but this is
> acceptible)
> - JFS adds a (small?) performance hit on top of the LVM overhead.
> - Does JFS add any performance gains via read-ahead and caching that
> aren't realized by raw LV AND are actually useful given the way ADSM
> accesses the volumes (the buffpool and logpool show that the db and log
> is doing its own optimization for disk access)

If you mirror your disk storage pool volumes, you want to use JFS.
Using AIX mirroring with a raw logical volume is bad because you
will clobber the mirror synchronization data that is kept in the
first bit of the logical volume.

> 3) Use AIX low level striping (software RAID 1) where volume size
> permits
> - I know I can 'dd' (sequential read & write) to SSA striped at close to
> 35 MB/s vs what 7-9 MB/s for single disk access; does this benifit ADSM
> given its access patterns?
> - alternately, does one give one volume to each spindle and let ADSM's
> spreading of load across multiple volumes do the trick?

If you are not mirroring your disk storage pool volumes, you want one
volume per spindle.  That way you don't lose as much if you lose a disk.

Striping is designed to maximize your sequential I/O bandwidth.  It
can give you a substantial performance penalty with random I/O by
spoiling the locality of reference that the application or filesystem
may provide.  For ADSM, I would be cautious with low-level striping
unless more information was available to base a decision on.
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