At 04/10/2017 11:34 PM, David Sterba wrote:
On Mon, Apr 10, 2017 at 10:17:52AM -0400, Josef Bacik wrote:
On Apr 9, 2017, at 11:27 PM, Qu Wenruo <quwen...@cn.fujitsu.com> wrote:
Hi,
Recent btrfs/137 test case makes me wonder what's the designed behavior of
btrfs inline data extent.
The current behavior in fact is quite a chaos.
We need a standard of how inline extent should behave.
1) max_inline limit
The problem of current max_inline is, it's never clear what it is
limiting.
For example, we don't allow page sized inline extent if not
compressed.
But we allow page sized inline extent if it's compressed.
Is it just limiting size after compression?
What if we really want to limit size before compression?
max_inline is for the actual space on disk. Compression takes up less
space, therefore you can fit bigger actual data into the inline area.
But in practice the other limits apply so we never inline file larger
than sectorsize. So the percieved behaviour is more like it's limit of
the file size, not the actual storage.
+1 for file size here.
Although both makes sense, the file size limit cause less confuse and
easier to understand.
2) inline extent condition
Is inline extent allowed if we have following regular extent?
For plain extent, prealloc can cause regular extent to co-exist with
inlined one.
While normal write will only convert inlined extent to regular one.
While for compressed extent, it can co-exist with regular extent, by
# xfs_io -f -c "pwrite 0 4k" -c sync -c "pwrite 4k 16k" /mnt/btrfs/file
So which is the correct behavior?
Personally I think we should not allow co-exist, as it's already
causing a lot of fixes for it, that's to say neither current
behavior is correct.
Historically we didn't have [inline][regular] because inline was
always < block size, so any change to the inline extent to extend it
resulted in a regular extent. Obviously that changed with fallocate,
so it is perfectly reasonable to have [inline][regular extent]
Even without fallocating, compression also makes difference.
# xfs_io -f -c "pwrite 0 4K" -c sync -c "pwrite 4k 8K" -c sync
/mnt/btrfs/file
Without compression, it causes one 12K extent.
With compression, it causes one inline extent and one 8K compressed extent.
Furthermore, even for compression, the extent layout change if the first
write is smaller than 4K.
# xfs_io -f -c "pwrite 0 4K" -c sync -c "pwrite 4k 8K" -c sync
/mnt/btrfs/file
^^^ This will cause inline extent with regular compressed extent.
# xfs_io -f -c "pwrite 0 2K" -c sync -c "pwrite 4k 8K" -c sync
/mnt/btrfs/file
^^^ While this will cause one compressed regular extent without inlined one
At least this behavior is confusing.
I'm not sure it's perfectly reasonable, makes things confusing. Does all
the extent handling code expect another extent after an inline?
Not really until recent.
For example, send can't handle it (at least not at best practice) until
this patch:
https://patchwork.kernel.org/patch/9667783/
And such inline-then-regular can even cause read corruption, fixed by
this one:
https://patchwork.kernel.org/patch/9449103/
And even before it, such layout can cause -EIO when reading:
https://patchwork.kernel.org/patch/9137293/
So it has been proven to be bug prone.
In my understanding, more from the user's perspective, is that inline
extent covers entire file smaller than some limit, otherwise it's all
regular extents.
+1 for all inline or all regular.
3) inline extent and fallocate
For inline extent, as long as we are calling fallocate inside the
page size, only the isize is expanded.
Only beyond page size, we get prealloc extents.
(However inlined extent is still here, not converted)
What's the designed behavior? Convert inline to regular or just
leave it as is?
Leave it.
"Convert."
fallocate doesn't change anything about existing regular
extents. Calling fallocate on a range completely inside of a regular
extent does nothing, why would this change with an inline extent?
But at least the nbytes is not correct.
# xfs_io -f -c "pwrite 0 2K" -c sync -c "falloc 2k 2k" -c sync
/mnt/btrfs/file1
The nbytes of that inode is still 2K, not 4K.
Thanks,
Qu
Because this leads to unexpected extent layout, contradicting what we've
told users for a long time. Inline + regular does not bring anything
special anyway.
Now
past the inline extent you get a new extent, exactly the same behavior
as a regular extent. Thanks,
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