G'day All. 

I’m trying to select the appropriate disk spindle speed for a proposal and 
would welcome any experience and opinions  (e.g. has anyone actively chosen 
10k/15k drives for a new ZFS build and, if so, why?).    
This is for ZFS over NFS for VMWare storage ie. primarily random 4kB read/sync 
writes (SLOG) + some general CIFS file serving.     About 40/60 read/write 
ratio.

The primary drive options I’m trying to compare are 48xHP SAS 500gb 7.2k(avg 
8ms seek - approx 80 random IOPS/drive) or 24xHP SAS 450g or 600g 10k drives 
(avg 4ms seek - approx 138 random IOPS/drive) which work out pretty close in 
price.

Ok, first theory.  Assuming sequential writes, the 7200 drives should be up to 
75% (at worst 80/138%) the IOPS of the 10k and with twice the number of 
spindles and a low latency ZIL SLOG that should give much better write 
performance**.  Correct?    What IOPS are people seeing from 7200 (approx 8ms 
avg seek) drives under mainly write loads?

Random reads IOPS are about the same on both options in terms of £/Random IO so 
the only problem is higher latency for reads that miss the ARC/L2ARC and are 
serviced by the 7200’s (avg 12.3- max 25.8ms) which is slower than the 10k 
would be (avg 7ms – max 14ms).  I’m currently planning 2x240GB L2ARC so 
hopefully we’ll be able to get a lot of the active read memory into cache and 
keep the latencies low.  Any suggestions how to identify the amount of “working 
dataset” on windows/netapp etc? 

I note ZFSBuild said they’ld do their next build with 15k SAS but I couldn’t 
follow their logic.   Anything else I’m missing.  

** My understanding is that  ZFS will adjust the amount of data accepted into 
each “transaction” (TXG) to ensure it can be written to disk in 5s.    Async 
data will stay in ARC, Sync data will also go to ZIL or, if overthreshold, will 
go to disk and pointer to ZIL(on low latency SLOG) – ie. all writes apart from 
sync writes over threshold will be unaffected by disk write latency from a 
client perspective.    Therefore if, for the same budget, 7200rpm can give you 
a higher iops, high latency disk whereas 10k gives you lower latency but lower 
iops, the 7200rpm system would end up providing highest write iops at lowest 
latency (due to SLOG).
-- 
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