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Re: Wrong file position after writing 65537 bytes to block device




On Tue, Dec 19, 2017 at 4:13 PM, Eric Blake <eblake@xxxxxxxxxx> wrote:
> Can block devices report an unaligned offset to lseek()?  If not, then when
> writing an unaligned value to a block device, don't we have to do a
> read-modify-write of the larger aligned cluster, and then put lseek() back
> to the unaligned boundary, and have extra magic in ftell() to track that we
> are at an unaligned position within the block device?  But that sounds like
> a lot of nasty overhead; and that it would be better to make sure that block
> devices can report unaligned lseek() locations (caveat: I haven't tested
> what Linux does in that regards).

>From what I observe on Linux, it supports writing at any offset to the
block device because it does a read-modify-write behind the scenes,
with accompanying nasty overhead (e.g. writes going at 64MB/s instead
of an "expected" 180MB/s).

I think you can observe this behavior on Linux by piping this
program's stdout to a block device (note: must be python3, not
python2):

#!/usr/bin/python3

import sys

block = b" " * 4096
while True:
    sys.stdout.buffer.write(block)
    sys.stdout.buffer.write(b" ")

and watching the block device activity with `dstat -d -D sdN` - you
should see a lot of reads.

Ivan

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