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Performance questions with SSD Cache

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Performance questions with SSD Cache

Postby craggy » Wed May 14, 2014 9:35 pm

We have a starwind V8 server as an iScsi datastore for our backup software with dual 10Gbe links using MPIO to ESXi 5.5 servers.

We have 12x 4TB Sata drives in Raid 10 and a 480GB SSD as flash cache for writes as we will have a lot of small random IO writes during our backup window.

We configured the storage device in Starwind to thick provisioned with 8GB L1 Write-back cache and 200GB L2 Write-through L2 cache on the SSD for writes.
Is this the optimal configuration for what we are trying to do?

I also have a concern about the SSD cache being a bottle neck on performance but I maybe way off in my assumption.
Basically the SSD can read/write at about 400/450MBs and the array can read/write at about 650/500MBs to the 12 disks for sequential stuff( small files will be much slower on the 12 disk array than the SSD I imagine).

When reading or writing to the array, will the maximum speed be limited by the speed of the SSD or the speed of the array when SSD is used as L2 cache?
When I run tests I observed that as data is read from the array it is written to the SSD as cache at the same time so it can be read back from SSD later instead of going to the array if it's busy the SSD will return the data quicker.
Problem is that I can't seem to read from the array any faster than 400MBs for big sequential files which I expected would have come from the array at roughly 650MBs.

Seems that the upper performance limit is pinned to that of the SSD regardless of the array performance capability.

Am I wrong here?
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Re: Performance questions with SSD Cache

Postby Max (staff) » Thu May 15, 2014 5:02 pm

Hi Craggy,
The only way to test this is to disable the L2 cache and do one more test.
I can't tell if it's the bottleneck for sure since you also have your RAM cache sitting on top, and the network which can slightly increase the latency.
Max Kolomyeytsev
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Re: Performance questions with SSD Cache

Postby craggy » Fri May 16, 2014 12:54 pm

Is there a way to disable L1/L2 cache without having to delete/recreate the device and add it back to the target because this causes re signature problem is ESXi
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Re: Performance questions with SSD Cache

Postby anton (staff) » Mon May 19, 2014 5:55 pm

ESXi problems should be pinpointed and fixed instead of a disabling cache. Please provide some more feedback and we'll be happy to look @ this issue. Thanks!

craggy wrote:Is there a way to disable L1/L2 cache without having to delete/recreate the device and add it back to the target because this causes re signature problem is ESXi
Regards,
Anton Kolomyeytsev

Chief Technology Officer & Chief Architect, StarWind Software

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