Re: Adding compression support for bluestore.

[Igor Fedotov <ifedotov@mirantis.com>]

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.