On Dec 7, 2016, at 2:52 AM, Renaud Mariana <rmariana@xxxxxxxxxx> wrote: > > Here are my answers, hope it will help solve this issue, thanks. > > Recap: > dpkg kibana on ext4 over a nbd device takes 10 minutes > with xfs it's only 30s. > with ext4 no extends only 30s. > > > kernels : > 4.5.7 has this issue as older kernel like 4.4.34 > The issue is also when nbd client & server run on same host > > > How small are the files? > here is the histogram of file sizes : http://pasteboard.co/6HC3nKyk2.png > We can see 5000 files around 512 Bytes. Definitely there is no value to use fallocate for 512-byte files, or any of the files that can be written in a single write() syscall. I'd expect any reasonable tool to be using a write buffer of at least 2-4MB these days to get good performance, so writes below the buffer size shouldn't use fallocate() at all. > dpkg using fallocate() ? > Yes, there are 16044 calls by the same process > what are these uninitialized extents ? Uninitialized extents are preallocated ranges of a file on disk that will read back as zero, but are not necessarily zero-filled at allocation time. For large files that are written randomly (or written slowly and may have contention from other writers) fallocate() + uninitialized extents will preallocate the space for the file so that it is (largely) contiguous on disk and overwrites will not result in random block allocation. Cheers, Andreas
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