ADSM-L

Performance/striping/etc.

1998-09-03 08:52:48
Subject: Performance/striping/etc.
From: Bruce Elrick <belrick AT HOME DOT COM>
Date: Thu, 3 Sep 1998 06:52:48 -0600
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)

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?

4) Put which disks nearest to the SSA adapter card; logs first, then db,
then stgpool?  Or db, log, then stgpool.

Food for thought...and any input/debate is appreciated.

Bruce
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