Le 26 févr. 07 à 18:30, Frank Cusack a écrit :
On February 26, 2007 9:05:21 AM -0800 Jeff Davis
<[EMAIL PROTECTED]> wrote:
That got me worried about the project I'm working on, and I wanted to
understand ZFS's caching behavior better to prove to myself that the
problem wouldn't happen under ZFS. Clearly the block will be in
cache on
the second read, but what I'd like to know is if ZFS will ask the
disk to
do a long, efficient sequential read of the disk, or whether it will
somehow not recognize that the read is sequential because the
requests
are coming from different processes?
ISTM zfs would be process-independent wrt that kind of decision. I
have
no clue about it but I couldn't imagine otherwise.
But you have to be aware that logically sequential reads do not
necessarily translate into physically sequential reads with zfs. zfs
translates all writes into sequential writes, ie writes to disk are
time ordered not data ordered (like log structured filesystems), so
whether or not the bits on disk are physically sequential depends on
HOW the file was written.
If Thomas is reading, I wonder how this affects mt-tar? Well, not
tar itself, but subsequent read of the data tar has written.
In each transaction, all blocks that belong to one file are treated
together before handling the next file.
So I think an mt-hot tar should not have too much of an adverse
effect on the on-disk layout.
-frank
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