Re: Adding compression support for bluestore.

[Sage Weil <sage@newdream.net>]

On Mon, 21 Mar 2016, Igor Fedotov wrote:
> On 19.03.2016 6:14, Allen Samuels wrote:
> > If we're going to both allow compression and delayed overwrite we simply
> > have to handle the case where new data actually overlaps with previous data
> > -- recursively. If I understand the current code, it handles exactly one
> > layer of overlay which is always stored in KV store. We need to generalize
> > this data structure. I'm going to outline a proposal, which If I get wrong,
> > I beg forgiveness -- I'm not as familiar with this code as I would like,
> > especially the ref-counted shared extent stuff. But I'm going to blindly
> > dive in and assume that Sage will correct me when I go off the tracks -- and
> > therefore end up learning how all of this stuff REALLY works.
> > 
> > I propose that the current bluestore_extent_t and bluestore_overlay_t  be
> > essentially unified into a single structure with a typemark to distinguish
> > between being in KV store or in raw block storage. Here's an example: (for
> > this discussion, BLOCK_SIZE is 4K and is the minimum physical I/O size).
> > 
> > Struct bluestore_extent_t {
> >     Uint64_t logical_size;			// size of data before any
> > compression. MUST BE AN INTEGER MULTIPLE of BLOCK_SIZE (and != 0)
> >     Uint64_t physical_size;                              // size of data on
> > physical media (yes, this is unneeded when location == KV, the
> > serialize/deserialize could compress this out --  but this is an unneeded
> > optimization
> >     Uint64_t location:1;                                    // values (in
> > ENUM form) are "KV" and "BLOCK"
> >     Uint64_t compression_alg:4;                  // compression algorithm...
> >     Uint64_t otherflags:xx;                             // round it out.
> >     Uint64_t media_address;                        // forms Key when
> > location == KV block address when location == BLOCK
> >     Vector<uint32_t> checksums;              // Media checksums. See
> > commentary on this below.
> > };
> > 
> > This allows any amount of compressed or uncompressed data to be identified
> > in either a KV key or a block store.
> > 
> As promised please find a competing proposal for extent map structure. It can
> be used for handling unaligned overlapping writes of both
> compressed/uncompressed data. It seems it's applicable for any compression
> policy but my primary intention was to allow overwrites that use totally
> different extents without the touch to the existing(overwritten) ones. I.e.
> that's what Sage explained this way some time ago:
> 
> "b) we could just leave the overwritten extents alone and structure the
> block_map so that they are occluded.  This will 'leak' space for some
> write patterns, but that might be okay given that we can come back later
> and clean it up, or refine our strategy to be smarter."
> 
> Nevertheless the corresponding infrastructure seems to be applicable for
> different use cases too.
> 
> At first let's consider simple raw data overwrite case. No compression,
> checksums, flags at this point for the sake of simplicity.
> Block map entry to be defined as follows:
> OFFS:  < EXT_OFFS, EXT_LEN, X_OFFS, X_LEN>
> where
> EXT_OFFS, EXT_LEN - allocated extent offset and size, AKA physical address and
> size.
> X_OFFS - relative offset within the block where valid (not overwritten) data
> starts. Full data offset = OFFS + X_OFFS
> X_LEN - valid data size.
> Invariant: Block length == X_OFFS + X_LEN
> 
> Let's consider sample block map transform:
> --------------------------------------------------------
> ****** Step 0 (two incoming writes of 50 Kb at offset 0 and 100K):
> ->Write(0,50)
> ->Write(100, 50)
> 
> Resulting block map ( OFFS: {EXT_OFFS, EXT_LEN, X_OFFS, X_LEN}  ):
> 0:      {EO1, 50, 0, 50}
> 100: {EO2, 50, 0, 50}
> 
> Where EO1, EO2 - physical addresses for allocated extents.
> Two new entries have been inserted.
> 
> ****** Step 1 ( overwrite that partially overlaps both existing blocks ):
> ->Write(25,100)
> 
> Resulting block map ( OFFS: {EXT_OFFS, EXT_LEN, X_OFFS, X_LEN}  ):
> 0:      {EO1, 50, 0, 25}
> 25:    {EO3, 100, 0, 100}
> 125: {EO2, 50, 25, 25}
> 
> As one can see new entry at offset 25 has appeared and previous entries have
> been altered (including the map key (100->125) for the last entry). No
> physical extents reallocation took place though - just a new one at E03 has
> been allocated.
> Please note that client accessible data for block EO2 are actually stored at
> EO2 + X_OFF(=25) and have 25K only despite the fact that extent has 50K total.
> The same for block EO1 - valid data length = 25K only.
> 
> 
> ****** Step 2 ( overwrite that partially overlaps existing blocks once again):
> ->Write(70, 65)
> 
> Resulting block map ( OFFS: {EXT_OFFS, EXT_LEN, X_OFFS, X_LEN}  ):
> 0:      {EO1, 50, 0, 25}
> 25:    {EO3, 100, 0, 45}
> 70:    {EO4, 65, 0, 65}
> 135: {EO2, 50, 35, 15}
> 
> Yet another new entry. Overlapped block entries at 25 & 125 were altered.
> 
> ****** Step 3 ( overwrite that partially overlaps one block and totally
> overwrite the last one):
> ->Write(100, 60)
> 
> Resulting block map ( OFFS: {EXT_OFFS, EXT_LEN, X_OFFS, X_LEN}  ):
> 0:      {EO1, 50, 0, 25}
> 25:    {EO3, 100, 0, 45}
> 70:    {EO4, 65, 0, 35}
> 100: {EO5, 60, 0, 60}
> -140: {EO2, 50, 50, 0}  -> to be removed as it's totally overwritten ( see
> X_LEN = 0 )
> 
> Entry for EO4 have been altered and entry EO2 to be removed. The latter can be
> done both immediately on map alteration and by some background cleanup
> procedure.
> 
> ****** Step 4 ( overwrite that totally overlap the first block):
> ->Write(0, 25)
> 
> Resulting block map ( OFFS: {EXT_OFFS, EXT_LEN, X_OFFS, X_LEN}  ):
> 0:      {EO6, 25, 0, 25}
> - 0:      {EO1, 50, 25, 0}  -> to be removed
> 25:    {EO3, 100, 0, 45}
> 70:    {EO4, 65, 0, 35}
> 100: {EO5, 60, 0, 60}
> 
> Entry for EO1 has been overwritten and to be removed.
> --------------------------------------------------------------------------------------
> 
> Extending this block map for compression is trivial - we need to introduce
> compression algorithm flag to the map. And vary EXT_LEN (and actual physical
> allocation) depending on the actual compression ratio.
> E.g. with ratio=3 (60K reduced to 20K) the record from the last step turn into
> :
> 100: {EO5, 20, 0, 60}
> 
> Other compression aspects handled by the corresponding policies ( e.g. when
> perform the compression ( immediately, lazily or totally in background ) or
> how to merge neighboring compressed blocks ) probably don't impact the
> structure of the map entry - they just shuffle the entries.

This is much simpler!  There is one case we need to address that I don't 
see above, though.  Consider,

- write 0~1048576, and compress it
- write 16384~4096

When we split the large extent into two pieces, the resulting extent map, 
as per above, would be something like

0:      {EO1, 1048576, 0, 4096, zlib}
4096:   {E02, 16384, 0, 4096, uncompressed}
16384:	{E01, 1048576, 20480, 1028096, zlib}

...which is fine, except that it's the *same* compressed extent, which 
means the code that decides that the physical extent is no longer 
referenced and can be released needs to ensure that no other extents in 
the map reference it.  I think that's an O(n) pass across the map when 
releasing.

Also, if we add in checksums, then we'd be duplicating them in the two 
instances that reference the raw extent.

I wonder if it makes sense to break this into two structures.. one that 
lists the raw extents, and another that maps them into the logical space.  
So that there is one record for {E01, 1048576, zlib, checksums}, and then 
the block map is more like

0:      {E0, 0, 4096}
4096:   {E1, 0, 4096}
16384:	{E0, 20480, 1028096}

and

0: E01, 1048576, 0, 4096, zlib, checksums
1: E02, 16384, 0, 4096, uncompressed, checksums

?

sage