On 18/02/2012 13:48, Da Rock wrote: > I was thinking along the lines of continuous heavy load of writing (some > read) rather large files (5G+ would be average - multiple!) - does that > warrant caching or is it only lots of smaller files? That and lots of > ~0.5G files (read mostly) is what defines the main load on the system. > > I ask because I'm not 100% sure of what the caching is for. I had > thought it was like the journal log for fast writing to be later written > to the filesystem itself, but now I think I may be wrong in my > judgement. It now sounds like a fast access for usual suspects. > > Now you see how a terabyte and a half disk space can be used in a matter > of hours :)
Right. That's a lot more file IO than I anticipated in my previous
answer. For that amount of usage, 8GB would definitely be required and
quite possibly more. Separate devices for ZIL and ARC would be a good
idea. (ZIL is effectively the caching for the write path, ARC for the
read path. That's a gross over-simplification actually, but good enough.)
The caching is vital -- it's where all the stuff like checking the
parity for a RAIDZn device happens, or the compression/decompression
actions. Yes, it works like file system journalling too.
Cheers,
Matthew
--
Dr Matthew J Seaman MA, D.Phil. 7 Priory Courtyard
Flat 3
PGP: http://www.infracaninophile.co.uk/pgpkey Ramsgate
JID: [email protected] Kent, CT11 9PW
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