Date: Sat, 18 Feb 2012 14:28:48 +0000 From: Matthew Seaman <m.seaman@infracaninophile.co.uk> To: freebsd-questions@freebsd.org Subject: Re: /usr/home vs /home Message-ID: <4F3FB5A0.9020806@infracaninophile.co.uk> In-Reply-To: <4F3FAC17.8000300@herveybayaustralia.com.au> References: <4F3ECF23.5000706@fisglobal.com> <20120217234623.cf7e169c.freebsd@edvax.de> <3D08D03C85ACFBB1ABCDC5DA@mac-pro.magehandbook.com> <alpine.BSF.2.00.1202172316230.11247@abbf.6qbyyneqvnyhc.pbz> <20120218112252.772c878b.freebsd@edvax.de> <4F3F80FD.8070201@herveybayaustralia.com.au> <4F3F8A46.1090908@infracaninophile.co.uk> <4F3F8D39.80907@herveybayaustralia.com.au> <4F3FA9FB.7030203@infracaninophile.co.uk> <4F3FAC17.8000300@herveybayaustralia.com.au>
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On 18/02/2012 13:48, Da Rock wrote:
> I was thinking along the lines of continuous heavy load of writing (som=
e
> 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.
>=20
> 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 writte=
n
> 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.
>=20
> Now you see how a terabyte and a half disk space can be used in a matte=
r
> 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
--=20
Dr Matthew J Seaman MA, D.Phil. 7 Priory Courtyard
Flat 3
PGP: http://www.infracaninophile.co.uk/pgpkey Ramsgate
JID: matthew@infracaninophile.co.uk Kent, CT11 9PW
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