vlho
Hot Shot
Hot Shot

Solution for problem with very slow writing on IBM DS3400

My enviroment:

Storage IBM DS3400 Single Controller (last firmware)

4x HDD 300GB-15K SAS (in RAID5)

Server IBM xSeries 236 (last bios)

FC card Emulex LP11000 (last firmware)

I tested VMware ESX 3.5 U2 and U3 with latest patches

Problem:

writing speed on datastore was very slow, typicaly 6 - 8MB/s

reading speed was OK, i.e. min. 25MB/s

In Storage Management - Subsystem Profile - Logical Drives was this information:

Read cache - Enabled

Write cache - Enabled (Suspended)

Write cache without batteries - Disabled

Write cache with mirroring - Enabled

Flush write cache after (in seconds) - 10.00

Dynamic cache read prefetch - Enabled

I run this script:

set logicaldrive mirrorEnabled=false;

and alleluia,

write speed is now as far as 32MB/s ...

Status write cache isn't suspended now.

I'm not sure but I think that "write cache suspended" is default setup at DS3400 with Single Controller...

It is true? What is your status of write cache?

P.S.

I am looking for command to disable Dynamic cache read prefetch too ...

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4 Replies
LanceYYC
Contributor
Contributor

We are having this exact same issue, I pulled the support logs and opened a ticket with IBM to see what they said.

The IBM tech confirmed that on the single controller DS3400, write caching even if 'enabled' will be 'suspended' by default since there is a chance for loss of data if the controller itself fails. In a dual controller system the two controllers will mirror each others caches so if you lose a controller it can still write what is in cache from the other controller he said. For the single controller it suspends write caching by default since there is no where to mirror it.

We are likely going to be buying a 2nd controller now for our DS3400, I wish IBM told us about this when we bought it, the performance of a SAN without write caching is pretty BAD Smiley Happy

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LanceYYC
Contributor
Contributor

Also the manual for the CLI is at ftp://ftp.software.ibm.com/systems/support/storage_disk_pdf/gc52127501.pdf

What is the benefit to disable the dynamic prefetch?

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vlho
Hot Shot
Hot Shot

SAN performance is resolved by enabling write back cache at the controller level and disabling prefetch on the SAN side:

http://kb.vmware.com/selfservice/microsites/search.do?cmd=displayKC&docType=kc&externalId=1002725

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Hiney
Enthusiast
Enthusiast

I know this is old, but let's record it for posterity

/opt/IBM_DS/client/SMcli da01a da01b -c "set logicaldrive cacheReadPrefetch=false;" -p password

"I have an inferiority complex, it's just not a very good one."

"I have an inferiority complex, it's just not a very good one."
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