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February 11th, 2015 20:00
VNXe3200 cannot use fastcache on NFS Exports
Hi,
i'd like to have your opinion on this matter.
We have a 3200 happily running off NFS exports connected to various ESXi 5.x hosts. Everything looks good but the fact that (EMC confirmed this) is not using the fastcache.
There are a bunch of SSD drives configured as fastcache (not FastVP) and assigned on top of the only pool available.
Further investigation shows that apparently due to a mechanism called "sync-writes" the block never hit the SSD as they are written direclty to the SAS drive, decreasing a lot the IOPS delivered by the system.
Now:
- vmware blames EMC saying that this setting is at storage level (and i am more inclined to think this way as there are other systems, like netapp, where you can force async at the export level).
- EMC blames Vmware saying that is ESXi problem that forces sync writes, even that there is not such option.
I have another VNX5300 running off NFS with ESXi on top that has fast cache and NFS datastores, and works perfectly (cache fully used) so i am inclined to think that this is a limitation of the VNXe3200 software. Fastcache documentation says clearly that as long as a block is read/written 3 times is moved to the cache, it doesnt mention anything about sync-writes or similar.
I really dont want to migrate the whole thing back to iscsi.
thoughts?
Nathan
sameek
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February 18th, 2015 05:00
Hi Nathan,
*Sync-writes as far as I am concerned is a feature that VMware use to make sure the data is protected at the highest level.
The ack is sent after making sure that the data is written to the disk and not from the cache( the normal ack is sent from cache rather than the data written to the drive) .
With FAST the data of 64k that are hit during a certain period of time is considers as a HOT data and is COPIED to the FLASH drives
And once after the data is moved from the initiator of the host to the target , then the (considering if it is a read req) then the data is checked at the DRAM level and then it moves to the FAST if it is unable to find the data block.
If the data is in the FAST cache, then the data is read/written from there.
The job does not end there, the ack is sent only after the data is written from FAST to the DISK.
So there is a delay.
The below explains more about it.
Also check page 7 , it would help.
https://support.emc.com/docu57829_Introduction-to-the-VNXe3200-FAST-Suite.pdf?language=en_US
I would like to know how you came to the conclusion that the SAS drives are hit and not your FAST drives, what are the drives you use and what is the configuration would be a useful info we could use.
pkilian-chipmancorp.com
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February 23rd, 2015 08:00
Sameeksha,
Pardon me for not understanding your response, but is fast cache used or not with an NFS share being used as VMWare storage. I ask because I have a VNXe3100 in production with 10 300 GB 15K drives and I have a VNXe3200 with 10 600 GB 10K drives with 200 GB of fast cache that I am ready to move into production to replace the VNXe3100. Even though it has the same number of slower drives as the VNXe3100 has, I was told that performance would be much better because of the flash cache. I only use NFS.
Best Regards.
NathanUS
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February 23rd, 2015 14:00
Hi,
We had someone else from EMC having a look. The NAS is actually working CORRECTLY, and the EMC engineer said that is not sure why the other engineer said that, which is obviously wrong. He showed us the stats and confirmed that cache has been used in both write/read.
Just for info, Vmware does not force anything as write sync, the setting is a property of the NAS server. EMC also does not use that mechanism as it auto-tier the pool (Vmware has no idea of what is happening).
Also pdkilian, i might be wrong but i am pretty sure VNXe31xx series DO NOT support fast cache. I am pretty sure you can install a bunch of SSD drives, but they cannot be assigned as fastcache to another pool, they probably can be used as normal disks. This is because the major feature of the VNXe3200 was in fact fastcache, something that was not present on older gen VNXes. Doublecheck with your supplier.
Nathan
pkilian-chipmancorp.com
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February 23rd, 2015 15:00
Nathan,
You are correct, the 3100 does not use fast cache, in fact, I don't think you can even use an SSD in the 3100. The VNXe3200 that I am currently testing has the fast cache in lieu of more/faster drives of my old VNXe3100. I was concerned that I would have to revist my VNXe3200 config before I put it into productions.
Best Regards.