Barriers and journaling filesystems [LWN.net]

Barriers and journaling filesystems

Posted May 25, 2008 2:47 UTC (Sun) by giraffedata (subscriber, #1954)
In reply to: Barriers and journaling filesystems by dlang
Parent article: Barriers and journaling filesystems

but on modern disks you really don't know where the borders of a track are. it's really just an array of blocks

I don't know what "but" refers to here; I made a statement about "if I were a disk drive," and the disk drive definitely knows where the borders of the tracks are. And though I have no actual evidence of it, I would be very surprised if the disk drive did not use that knowledge to optimize performance.

The idea suggested in the article that ext3 tends to get ordered writes (to the platters) because it usually does journal updates in logical block number order seems to assume that the disk drive does writes from cache to platter in logical block number order. I can't believe that it would do that, even usually, because it would be so much slower than optimum.

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Barriers and journaling filesystems

Posted May 25, 2008 3:27 UTC (Sun) by dlang (✭ supporter ✭, #313) [Link]

the drive will write an entire track at a time (they have done so for years)

but you have no way of knowing if the journal that you allocated spans a track boundry (since
you don't know where the boundry is), and you also don't know if the drive has re-allocated
any blocks to avoid bad spots on the disk in the middle of your journal.

either one of these will destroy the atomic nature of writes to the journal

Barriers and journaling filesystems

Posted May 25, 2008 6:40 UTC (Sun) by giraffedata (subscriber, #1954) [Link]

Whether it writes the whole track doesn't matter. It's the order in which it writes the blocks. Does it wait for the beginning of the track to come around and write from start to finish (average time - 1.5 revolutions)? Or does it write starting now (average time 1 revolution)? I presume it's the latter.

Nobody's said anything about ext3 knowing where the blocks are physically, so I don't know why you bring that up. The article just says ext3 updates the journal in logical block number order and suggests that means they tend to go the platter in the same order.

And I've been saying I don't believe that.

Barriers and journaling filesystems

Posted May 25, 2008 10:53 UTC (Sun) by dlang (✭ supporter ✭, #313) [Link]

I misunderstood what you were saying. I thought that you were saying that if the entire
journal was on the same track it would get written all at once, even if it wrapped over the
end of the journal.