On 02/19/12 00:28, Matthew Seaman wrote:
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.
Thanks for the tips Matthew. Now you know why I haven't tackled that
hurdle yet :)
I'm gunna need some serious hardware...
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