I have a ESXi 5.5 server and I'm experiencing high write latency on a local datastore.
This datastore is on a virtual disk provided by a RAID card (two SATA disks on RAID 1).
When I copy large files, it takes ages to complete the transfer and write latency averages 84ms! This is way too much.
I know RAID 1 doesn't improve write rates, btw.
So I'm trying to find where the bottleneck is. Could it be the RAID card? (PCI-e 8x, 100% hardware). Could it be a fragmentation issue? (Not very probable on VMFS).
If you have already experienced high latency on a local datastore, I'd like to have your feedback. Thanks 🙂
Best Answer
SImply said your problem is:
Turn it how you like, but two likely very slow discs are 2 very slow discs and nothing except heavy caching will work around it. You have a small IOPS budget right there and the only thin that can fix that is having a larger one.
Example - using Raid 10 with 8-10 discs will give you a lot more IOPS. Using an enerprise Raid controller (like the Adaptec 71605Q) and putting in multiple SSD as transparently used cache will fix the write issue. I am regularly copying files with 500mb/s to a Raid 6 thanks to that.
But your problem is that 2 not fast (i.e. max. 7200 RPM) SATA discs are just that and you dont seem to have the other hardware to mitigate this.