Re: Feature requests: online backup - defrag - change RAID level

Chris Murphy
2019-09-14T18:29:25Z

On Fri, Sep 13, 2019 at 5:04 AM Austin S. Hemmelgarn
<ahferroin7@gmail.com> wrote:
>
> Do you have a source for this claim of a 128MB max extent size? Because
> everything I've seen indicates the max extent size is a full data chunk
> (so 1GB for the common case, potentially up to about 5GB for really big
> filesystems)

Yeah a block group can be a kind of "super extent". I think the
EXTENT_DATA maxes out at 128M but they are often contiguous, for
example

item 308 key (5741459 EXTENT_DATA 0) itemoff 39032 itemsize 53
generation 241638 type 1 (regular)
extent data disk byte 193851400192 nr 134217728
extent data offset 0 nr 134217728 ram 134217728
extent compression 0 (none)
item 309 key (5741459 EXTENT_DATA 134217728) itemoff 38979 itemsize 53
generation 241638 type 1 (regular)
extent data disk byte 193985617920 nr 134217728
extent data offset 0 nr 134217728 ram 134217728
extent compression 0 (none)
item 310 key (5741459 EXTENT_DATA 268435456) itemoff 38926 itemsize 53
generation 241638 type 1 (regular)
extent data disk byte 194119835648 nr 134217728
extent data offset 0 nr 134217728 ram 134217728
extent compression 0 (none)

Where FIEMAP has a different view (via filefrag -v)

ext: logical_offset: physical_offset: length: expected: flags:
0: 0.. 131071: 47327002.. 47458073: 131072:
1: 131072.. 294911: 47518701.. 47682540: 163840: 47458074:
2: 294912.. 360447: 50279681.. 50345216: 65536: 47682541:
3: 360448.. 499871: 50377984.. 50517407: 139424: 50345217: last,eof
Fedora-Workstation-Live-x86_64-31_Beta-1.1.iso: 4 extents found

Those extents are all bigger than 128M. But they're each made up of
contiguous EXTENT_DATA items.

Also, the EXTENT_DATA size goes to a 128K max for any compressed
files, so you get an explosive number of EXTENT_DATA items on
compressed file systems, and thus metadata to rewrite.

I wonder if instead of a rewrite of defragmenting, if there could be
improvements to the allocator to write bigger extents. I guess the
problem really comes from file appends? Smarter often means slower but
perhaps it could be a variation on autodefrag?

--
Chris Murphy