Re: getdents - ext4 vs btrfs performance

[Yongqiang Yang]

On Wed, Mar 14, 2012 at 4:12 PM, Lukas Czerner <lczerner@xxxxxxxxxx> wrote:
> On Tue, 13 Mar 2012, Ted Ts'o wrote:
>
>> On Tue, Mar 13, 2012 at 04:22:52PM -0400, Phillip Susi wrote:
>> >
>> > I think a format change would be preferable to runtime sorting.
>>
>> Are you volunteering to spearhead the design and coding of such a
>> thing?  Run-time sorting is backwards compatible, and a heck of a lot
>> easier to code and test...
>
> Actually I am in the process of creating some projects for the local
> universities and this certainly sounds like something that some student
> can do. I have already put that into the proposal, so we might have
> someone looking into this in the near future. Since the problem has been
> there since forever, I think that it's not that critical to solve it
> right away.
>
> I kind of like the idea about having the separate btree with inode
> numbers for the directory reading, just because it does not affect
> allocation policy nor the write performance which is a good thing. Also
> it has been done before in btrfs and it works very well for them. The
> only downside (aside from format change) is the extra step when removing
> the directory entry, but the positives outperform the negatives.
>
>
> Maybe this might be even done in a way which does not require format
> change. We can have new inode flag (say EXT4_INUM_INDEX_FL) which will
> tell us that there is a inode number btree to use on directory reads.
> Then the pointer to the root of that tree would be stored at the end of
> the root block of the hree (of course if there is still some space for
> it) and the rest is easy.
dx_root takes 32 bytes from 60 bytes of i_data.  The beginning of
dx_entries is controlled by info_length, that is, dx_root->info +
dx_root->info->info_length.   It seems that we can put the new tree
root behind dx_root and add the length of the new tree root to
info_length.  Then the old code can handle new filesystem and new code
can handle old filesystem because of the flag you described above.

If no one attempts to do it ,  I can do it as a google summer of code project.

Yongqiang.
>
> So If we mount the old file system with the new code, there will not be
> the flag, but with newly added directories it could be used anyway. Also
> e2fsck could easily add the inumber tree into the directory if it is
> missing.
>
> If we mount the new file system with the old code (the one which does
> not know about this feature), everything would stay the same and we
> would not touch the inumber tree at all. Of course there is a
> possibility that we would rewrite the end of the root htree, overwriting
> the pointer to the inumber tree and effectively losing one block. We
> would not actually care that we rewrote this information because we
> do not actually need it, so the only downside is the one block lost,
> which is not that bad I guess.
>
> Now, we have rewritten the pointer to the inumber tree and we're going
> to mount it with the new code again. It would expect the the inumber
> tree to be there, but not blindly. Some king of magic information would
> have to be stored in the inumber root pointer so we can be sure that it
> really is what we're looking for and if it is not there, well then we do
> not care and walk the directory in the old way. And we can alway create
> the inumber tree in the new directory. And again e2fsck should be able
> to fix that for us as well.
>
> So that is just an idea, I have not looked at the actual code to see it
> it would be really possible, but it certainly seems like so. Am I
> missing something ?
>
>
> Anyway, if there is going to be a format change I agree that having
> solution for those who are not comfortable with the format change is
> equally as important. But maybe there will be better solution which does
> not require the format change.
>
> Thanks!
> -Lukas
>
>>
>> The reality is we'd probably want to implement run-time sorting
>> *anyway*, for the vast majority of people who don't want to convert to
>> a new incompatible file system format.  (Even if you can do the
>> conversion using e2fsck --- which is possible, but it would be even
>> more code to write --- system administrators tend to be very
>> conservative about such things, since they might need to boot an older
>> kernel, or use a rescue CD that doesn't have an uptodate kernel or
>> file system utilities, etc.)
>>
>> > So the index nodes contain the hash ranges for the leaf block, but
>> > the leaf block only contains the regular directory entries, not a
>> > hash for each name?  That would mean that adding or removing names
>> > would require moving around the regular directory entries wouldn't
>> > it?
>>
>> They aren't sorted in the leaf block, so we only need to move around
>> regular directory entries when we do a node split (and at the moment
>> we don't support shrinking directories), so we don't have to worry the
>> reverse case.
>>
>> > I would think that hash collisions are rare enough that reading a
>> > directory block you end up not needing once in a blue moon would be
>> > chalked up under "who cares".  So just stick with hash, offset pairs
>> > to map the hash to the normal directory entry.
>>
>> With a 64-bit hash, and if we were actually going to implement this as
>> a new incompatible feature, you're probably right in terms of
>> accepting the extra directory block search.
>>
>> We would still have to implement the case where hash collisions *do*
>> exist, though, and make sure the right thing happens in that case.
>> Even if the chance of that happening is 1 in 2**32, with enough
>> deployed systems (i.e., every Android handset, etc.) it's going to
>> happen in real life.
>>
>>                                       - Ted
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-- 
Best Wishes
Yongqiang Yang
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