Messages in this thread | | | Date | Mon, 4 Mar 2002 23:22:52 -0800 (PST) | From | Jeremy Higdon <> | Subject | Re: [PATCH] 2.4.x write barriers (updated for ext3) |
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On Mar 4, 8:57am, James Bottomley wrote: > > > 2a) Are the filesystems asking for something impossible? Can drives > > really write block N and N+1, making sure to commit N to media before > > N+1 (including an abort on N+1 if N fails), but still keeping up a > > nice seek free stream of writes? > > These are the "big" issues. There's not much point doing all the work to > implement ordered tags, if the end result is going to be no gain in > performance.
If a drive does reduced latency writes, then blocks can be written out of order. Also, for a trivial case: with hardware RAIDs, when the data for a single command is split across multiple drives, you can get data blocks written out of order, no matter what you do.
I don't think a filesystem can make any assumptions about blocks within a single command, though with ordered tags (assuming driver and device support) and no write caching, it can make assumptions between commands.
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