Hi

I have been doing some extensive testing on the entire disk storage pool
question.

The first question, is what is the feed? i.e. how many network connections
does the TSM Server have. I am assuming all your larger systems are likely
to go LAN FREE direct to tape. Also, what volume are you backing up.

The disk geometry that I have used for testing is 146GB 15K FC, 300GB 10K
FC and the 500GB 7.5K SATA drives. All in RAID 5 configurations, optimised
for the array that they are in. Any cached disk subsystem, will cache the
Io writes and to an extent 'hide' disk performance from the host. However,
with TSM, the sustained high write profile through a backup window means
the disk array cache becomes filled with 'write pending' data when using
SATA drives. This is data in cache waiting to be destaged, i.e. waiting on
the physical spindles. The FC disks showed a write pending amount, but
once the array starts to favour the destage, the disks can keep up- whilst
retaining a good host performance- 4 times that of the SATA.

In this case the disk array usually has to throttle the host, and I have
been seeing 50%+ wait_io on the SAR output of the TSM server.

I personnally would advise, for larger systems do not use SATA. It's IO
profile is more suited to a low IO activuity and certainly not the
susteined IO that is seen during the TSM daily operation.

Also, bare in mind rebuild times on SATA. RAID 6 (dual parity) will slow
the write down even more, RAID 5 may present a 30 hour + period of
exposure to data in the pools.

Ian Smith
-------------------------------------------------------




Johnny Lea <jlea AT DIS.UMSMED DOT EDU>
Sent by: "ADSM: Dist Stor Manager" <ADSM-L AT VM.MARIST DOT EDU>
11/07/2007 19:01
Please respond to
"ADSM: Dist Stor Manager" <ADSM-L AT VM.MARIST DOT EDU>


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Subject
[ADSM-L] SATA disk?






Has anyone used SATA drives for primary storage.  Is it really a bad idea?








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