Sage, Allen et. al.
Please find some follow-up on our discussion below.
Your past and future comments are highly appreciated.
WRITE/COMPRESSION POLICY and INTERNAL BLUESTORE STRUCTURES OVERVIEW.
Used terminology:
Extent - basic allocation unit. Variable in size, maximum size is
limited by lblock length (see below), alignment: min_alloc_unit param
(configurable, expected range: 4-64 Kb .
Logical Block (lblock) - standalone traceable data unit. Min size
unspecified. Alignment unspecified. Max size limited by max_logical_unit
param (configurable, expected range: 128-512 Kb)
Compression to be applied on per-extent basis.
Multiple lblocks can refer specific region within a single extent.
POTENTIAL COMPRESSION APPLICATION POLICIES
1) Read/Merge/Write at initial commit phase. (RMW)
General approach:
New write request triggers partially overlapped lblock(s)
reading/decompression followed by their merge into a set of new lblocks.
Then compression is (optionally) applied. Resulting lblocks overwrite
existing ones.
For non-overlapping/fully overlapped lblocks read/merge steps are simply
bypassed.
- Read, merge and final compression take place prior to write commit ack
that can impact write operation latency.
2) Deferred RMW for partial overlaps. (DRMW)
General approach:
Non-overlapping/fully overlapped lblocks handled similar to simple RMW.
For partially overlapped lblocks one should use Write-Ahead Log to defer
RMW procedure until write commit ack return.
- Write operation latency can still be high in some cases (
non-overlapped/fully overlapped writes).
- WAL can grow significantly.
3) Writing new lblocks over new extents. (LBlock Bedding?)
General approach:
Write request creates new lblock(s) that use freshly allocated extents.
Overlapped regions within existing lblocks are occluded.
Previously existing extents are preserved for some time (or while being
used) depending on the cleanup policy.
Compression to be performed before write commit ack return.
- Write operation latency is still affected by the compression.
- Store space usage is usually higher.
4) Background compression (BCOMP)
General approach:
Write request to be handled using any of the above policies (or their
combination) with no compression applied. Stored extents are compressed
by some background process independently from the client write flow.
Merging new uncompressed lblock with already compressed one can be
tricky here.
+ Write operation latency isn't affected by the compression.
- Double disk write occurs
To provide better user experience above-mentioned policies can be used
together depending on the write pattern.
INTERNAL DATA STRUCTURES TO TRACK OBJECT CONTENT.
To track object content we need to introduce following 2 collections:
1) LBlock map:
That's a logical offset mapping to a region within an extent:
LOFFS -> {
EXTENT_REF - reference to an underlying extent, e.g. pointer
for in-memory representation or extent ID for "on-disk" one
X_OFFS, X_LEN, - region descriptor within an extent: relative
offset and region length
LFLAGS - some associated flags for the lblock. Any usage???
}
2) Extent collection:
Each entry describes an allocation unit within storage space.
Compression to be applied on per-extent basis thus extent's logical
volume can be greater than it's physical size.
{
P_OFFS - physical block address
SIZE - actual stored data length
EFLAGS - flags associated with the extent
COMPRESSION_ALG - An applied compression algorithm id if any
CHECKSUM(s) - Pre-/Post compression checksums. Use cases TBD.
REFCOUNT - Number of references to this entry
}
The possible container for this collection can be a mapping: id ->
extent. It looks like such mapping is required during on-disk to
in-memory representation transform as smart pointer seems to be enough
for in-memory use.
SAMPLE MAP TRANSFORMATION FOR LBLOCK BEDDING POLICY ( all values in Kb )
Config parameters:
min_alloc_unit = 4
max_logical_unit = 64
--------------------------------------------------------
****** Step 0 :
->Write(0, 50), no compression
->Write(100, 60), no compression
Resulting maps:
LBLOCK map ( OFFS: { EXT_REF, X_OFFS, X_LEN} ):
0: {EO1, 0, 50}
100: {EO2, 0, 60}
EXTENT map ( ID: { P_OFFS, SIZE, ALG, REFCOUNT} ):
EO1: { POFFS_1, 50, NONE, 1} //totally allocated 52 Kb
EO2: { POFFS_2, 60, NONE, 1} //totally allocated 60 Kb
Where POFFS_1, POFFS_2 - physical addresses for allocated extents.
****** Step 1
->Write(25, 100), compressed
Resulting maps:
LBLOCK map ( OFFS: { EXT_REF, X_OFFS, X_LEN} ):
0: {EO1, 0, 25}
25: {EO3, 0, 64} //compressed into 20K
79: {EO4, 0, 36} //compressed into 15K
125: {EO2, 25, 35}
EXTENT map ( ID: { P_OFFS, SIZE, ALG, REFCOUNT} ):
EO1: { POFFS_1, 50, NONE, 1} //totally allocated 52 Kb
EO2: { POFFS_2, 60, NONE, 1} //totally allocated 60 Kb
EO3: { POFFS_3, 20, ZLIB, 1} //totally allocated 24 Kb
EO4: { POFFS_4, 15, ZLIB, 1} //totally allocated 16 Kb
As one can see new entries at offset 25 & 79 have 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 new ones (EO3
& EO4) have been allocated.
Please note that client accessible data for block EO2 are actually
stored at P_OFFS_2 + X_OFF and have 35K only despite the fact that
extent has 60K total.
The same for block EO1 - valid data length is 25K only.
Extent EO3 actually stores 20K of compressed data corresponding to 64K
raw one.
Extent EO4 actually stores 15K of compressed data corresponding to 36K
raw one.
Single 100K write has been splitted into 2 lblocks to address
max_logical_unit constraint
****** Step 2
->Write(70, 65), no compression
LBLOCK map ( OFFS: { EXT_REF, X_OFFS, X_LEN} ):
0: {EO1, 0, 25}
25: {EO3, 0, 45}
70: {EO5, 0, 65}
-125: {EO4, 36, 0} -> to be removed as it's totally overwritten ( see
X_LEN = 0 )
135: {EO2, 35, 25}
EXTENT map ( ID: { P_OFFS, SIZE, ALG, REFCOUNT} ):
EO1: { POFFS_1, 50, NONE, 1} //totally allocated 52 Kb
EO2: { POFFS_2, 60, NONE, 1} //totally allocated 60 Kb
EO3: { POFFS_3, 20, ZLIB, 1} //totally allocated 24 Kb
-EO4: { POFFS_4, 15, ZLIB, 0} //totally allocated 16 Kb, can be
released as refcount = 0
EO5: { POFFS_5, 65, NONE, 1} //totally allocated 68 Kb
Entry at at offset 25 entry has been altered and entry at offset 125 to
be removed. The latter can be done both immediately on map alteration
and by some background cleanup procedure.
****** Step 3
->Write(100, 60), compressed to 30K
LBLOCK map ( OFFS: { EXT_REF, X_OFFS, X_LEN} ):
0: {EO1, 0, 25}
25: {EO3, 0, 45}
70: {EO5, 0, 65}
100: {EO6, 0, 60}
-160: {EO2, 60, 0} -> to be removed as it's totally overwritten ( see
X_LEN = 0 )
EXTENT map ( ID: { P_OFFS, SIZE, ALG, REFCOUNT} ):
EO1: { POFFS_1, 50, NONE, 1} //totally allocated 52 Kb
EO2: { POFFS_2, 60, NONE, 1} //totally allocated 60 Kb
EO3: { POFFS_3, 20, ZLIB, 1} //totally allocated 24 Kb
-EO5: { POFFS_5, 65, NONE, 0} //totally allocated 68 Kb, can be
released as refcount = 0
EO6: { POFFS_6, 30, ZLIB, 1} //totally allocated 32 Kb
Entry at offset 100 has been altered and entry at offset 160 to be removed.
****** Step 4
->Write(0, 25), no compression
LBLOCK map ( OFFS: { EXT_REF, X_OFFS, X_LEN} ):
0: {EO7, 0, 25}
-25: {EO1, 25, 0} -> to be removed
25: {EO3, 0, 45}
70: {EO5, 0, 65}
100: {EO6, 0, 60}
-160: {EO2, 60, 0} -> to be removed as it's totally overwritten ( see
X_LEN = 0 )
EXTENT map ( ID: { P_OFFS, SIZE, ALG, REFCOUNT} ):
-EO1: { POFFS_1, 50, NONE, 1} //totally allocated 52 Kb, can be
released as refcount = 0
EO2: { POFFS_2, 60, NONE, 1} //totally allocated 60 Kb
EO3: { POFFS_3, 20, ZLIB, 1} //totally allocated 24 Kb
EO6: { POFFS_6, 30, ZLIB, 1} //totally allocated 32 Kb
EO7: { POFFS_7, 25, None, 1} //totally allocated 38 Kb
Entry at offset 0 has been overwritten and to be removed.
IMPLMENTATION ROADMAP
1) Refactor current Bluestore implementation to introduce the suggested
twin-structure design.
This will support raw data READ/WRITE without compression. Major policy
to implement is lblock bedding.
As an additional option DRMW to be implemented to provide a solution
equal to the current implementation. This might be useful for
performance comparison.
2) Add basic compression support using lblock bedding policy.
This will lack most of management/statistics features too.
3) Add compression management/statistics. Design to be discussed.
4) Add check sum support. Goals and design to be discussed.
5) Add RMW/DRMW policies [OPTIONAL]
6) Add background task support for compression/defragmentation/cleanup.
Thanks,
Igor.
On 21.03.2016 18:50, Sage Weil wrote:
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
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