On 25.09.2015 17:14, Sage Weil wrote:
On Fri, 25 Sep 2015, Igor Fedotov wrote:
Another thing to note is that we don't have the whole object ready for
compression. We just have some new data block written(appended) to the object.
And we should either compress that block and save mentioned mapping data or
decompress the existing object data and do full compression again.
And IMO introducing seek points is largely similar to what we were talking
about - it requires a sort of offset mapping as well.
Probably compression at OSD has some Pros as well. But it wouldn't eliminate
the need to "muck with stripe sizes or anything".
I think the best option here is going to be to compress the "stripe unit".
I.e., if you have a stripe_size of 64K, and are doing k=4 m=2, then the
stripe unit is 16K (64/4). Then each shard has an independent unit it can
compress/decompress and we don't break the ability to read a small extent
by talking to only a single shard.
Sage, are you considering compression applied after erasure coding here?
Please note that one needs to compress additional 50% of data this way.
Generated 'm' chunks need to be processed as well.
And you lose an ability to perform recovery on OSD down without applying
decompression ( and probably another compression) to remaining shards.
Contrary doing compression before EC produces reduced data set for EC (
some CPU cycles saving) and is suitable for recovery procedure not
involving additional decompression/compression pair.
But I suppose 'stripe unit' from the above wouldn't work in this case -
compression entity has to produce blocks having "stripe unit" size.
This way you can fit all compressed data into single shard only. But
that's hard to achieve....
Thus as usual we should choose what drawbacks(benefits) are less(more)
important here:
ability to read small extent from single shard + increased data set for
compression vs. ability to omit total decompression on recovery +
reduced data set for EC.
*Maybe* the shard could compress contiguous stripe units if multiple
stripes are written together..
In any case, though, there will some metadata it has to track with the
object, because the stripe units are no longer fixed size, and there will
be object_size/stripe_size of them. I forget if we are already storing a
CRC for each stripe unit or if it is for the entire shard... if it's the
former then this won't be a huge change, I think.
sage
On 24.09.2015 20:53, Samuel Just wrote:
The catch is that currently accessing 4k in the middle of a 4MB object
does not require reading the whole object, so you'd need some kind of
logical offset -> compressed offset mapping.
-Sam
On Thu, Sep 24, 2015 at 10:36 AM, Robert LeBlanc <robert@xxxxxxxxxxxxx>
wrote:
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I'm probably missing something, but since we are talking about data at
rest, can't we just have the OSD compress the object as it goes to
disk? Instead of
rbd\udata.1ba49c10d9b00c.0000000000006859__head_2AD1002B__11 it would
be
rbd\udata.1ba49c10d9b00c.0000000000006859__head_2AD1002B__11.{gz,xz,bz2,lzo,etc}.
Then it seems that you don't have to muck with stripe sizes or
anything. For compressible objects they would be less than 4MB, some
of theses algorithms already say if it is not compressible enough,
just store it.
Something like zlib Z_FULL_FLUSH may help provide some seek points
within an archive to prevent decompressing the whole object for reads?
- ----------------
Robert LeBlanc
PGP Fingerprint 79A2 9CA4 6CC4 45DD A904 C70E E654 3BB2 FA62 B9F1
On Thu, Sep 24, 2015 at 10:25 AM, Igor Fedotov wrote:
On 24.09.2015 19:03, Sage Weil wrote:
On Thu, 24 Sep 2015, Igor Fedotov wrote:
Dynamic stripe sizes are possible but it's a significant change from
the
way the EC pool currently works. I would make that a separate project
(as
its useful in its own right) and not complicate the compression
situation.
Or, if it simplifies the compression approach, then I'd make that
change
first. sage
Just to clarify a bit. What I saw when played with Ceph. Please correct
me
if I'm wrong..
For low-level RADOS access client data written to EC pool has to be
aligned
with stripe size . The last block can be unaligned though but no more
appends are permitted in this case.
Data copied from cache goes in blocks up to 8Mb size. In general case
the
last block seems to have unaligned size too.
EC pool additionally performs alignment of the incoming blocks to stripe
bound internally. This way blocks going to EC lib are always aligned.
We should probably perform compression prior to this alignment.
Thus some dependency on stripe size is present in EC pools but it's not
that
strict.
Thanks,
Igor
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