I can't answer the majority of these -- but the high-use SSA volumes
should be farthest from the adapter (ie, in the middle of the loop). The
SSA spec calls for drives to pass on requests to other drives *before*
acting upon their own requests, so the heavy I/O activity should not be
nearest the SSA adapter.
And yes, this is counter-intuitive!
TomK
> ----------
> From: Bruce Elrick[SMTP:belrick AT HOME DOT COM]
> Sent: Thursday, September 03, 1998 7:52 AM
> To: ADSM-L AT VM.MARIST DOT EDU
> Subject: Performance/striping/etc.
>
> 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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