Neil Perrin wrote:
Under the hood in ZFS, writes are committed using either shadow
paging or
logging, as I understand it. So I believe that I mean to ask whether a
write(2), pushed to ZPL, and pushed on down the stack, can be split
into
multiple transactions? Or, instead, is it guaranteed to be committed
in a
single transaction, and so committed atomically?
A write made through the ZPL (zfs_write()) will be broken into
transactions
that contain at most 128KB user data. So a large write could well be
split
across transaction groups, and thus committed separately.
So what happens if application is doing a synchronous write of lets say
512KB?
The write will be splitted in at least 4 separate transactions and the
write will be confirmed to the application only after all 512KB has been
written. But is there a possibility that if after a first transaction
was commited system crashed and although write was not confirmed to the
application 128KB of it has been commited to the disk? Or will it be
rolled back? Basically for synchronous writes of more than 128KB - is it
guaranteed that all data under a given write is committed or nothing at all?
--
Robert Milkowski
http://milek.blogspot.com
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