On 10/16/15 03:53, Bruce Evans wrote:
On Fri, 16 Oct 2015, Hans Petter Selasky wrote:
On 10/16/15 08:21, Bruce Evans wrote:
[Bruce Evans didn't write:]
In addition, making the file contiguous in LBA space doesn't
improve the access times from flash devices because they have no seek
time.
This is not exactly true, like Bruce pointed out too. Maybe there
should be a check, that if the block is too small reallocate it, else
leave it for the sake of the flash. Doing 1K accesses versus 64K
accesses will typically show up in the performance benchmark
regardless of how fast the underlying medium is.
Now I don't unerstand the whole point of the change. Anything that reduces
i/o's is good, but AFAIK ffs_doreallocblks() is all in software. Writes
should be delayed so that it doesn't have to do extra i/o's to back out of
committed writes. Often it reduces the number of writes and increases
their size by making blocks contiguous so that the write can be clustered.
Increasing the write size is especially good for flash devices, but maybe
ffs's default block size is already large enough.
I agree with Bruce: reallocation (which our ext2fs also does) happens
in memory, before it hits the disk.
By the nature of their load, Netflix doesn't care about fragmentation,
but even in that case reallocblk doesn't hurt, and I don't see anything
inherent in SSDs that makes fragmentation desirable.
Of course, no one understands reallocblk better than Kirk, and Warner
knows SSD's pretty well so I must be missing something. :).
Pedro.
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