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Date: Wed, 24 Mar 2010 10:54:14 +1100
From: Andrew Snow <als@modulus.org>
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To: Andriy Gapon <avg@freebsd.org>
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Cc: freebsd-fs@freebsd.org
Subject: Re: on st_blksize value
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Andriy Gapon wrote:

> One practical benefit can be with ZFS: if a filesystem has recordsize > PAGE_SIZE
> (e.g. default 128K) and it has checksums or compression enabled, then
> (over-)writing in blocks smaller than recordsize would require reading of a whole
> record first. 

Not strictly true: in ZFS the recordsize setting is for the maximum size 
of a record, it can still write smaller than this.  If you overwrite 1K 
in the middle of a 128K record then it should just be writing a 1K 
block.  Each block has its own checksum attached to it so there's no 
need to recalculate checksums for data that isn't changing.


- Andrew