Sage,
thanks a lot for detailed overview.
Below is a brief proposal on Extent Manager extension to support WAL.
Please tell me what's your final decision its fortune? Are there any
plans to use it or we completely rebase to the code you are woring on.
Is there any sense for me to to continue working on it or I should
switch to something in your stream.
Here is the idea that's pretty simple:
1) Add new method(s) to BlockOpInterface to add WAL record. EX to call
it when corresponding overwrite (or whatever other case) is detected.
2) Add method(s) to ExtentManger to process WAL record. They are to be
called from _wal_apply/_do_wal_op and EM to perform corresponding RMW
tasks. It does that using other regular BlockOpInterface methods (
read/write/allocate etc) thus Bluestore environment should be ready to
handle them during the WAL phase as it does during the initial write
hanlding one. But it looks like we need that anyway - EM doesn't create
any additional complexities here.
And a couple of general notes on WAL apply handling that I realized
recently and we should probably care of.
1) when WAL is applied on compressed blobs one should always allocate
and use a new blob. Even for the case when resulting buffer fit into the
existing blob. The rationale for that is that if we crash after the
write phase we and before KV update we can't properly execute WAL
replay. Source data is altered and we are unable to repeat RMW.
2) Similarly during WAL replay after the failure one should disable
checksum verification for existing data read if any. The rationale is
the same - we can crash after the write and before KV update thus
previous checksum in KV is invalid.
Thanks,
Igor.
On 11.05.2016 4:10, Sage Weil wrote:
On Tue, 10 May 2016, Sage Weil wrote:
Making the wal part of the consistency model is more complex, but it means
we can (1) log our intent to overwrite atomically with the kv txn commit,
and then (2) do the async overwrite. It will get a bit more complex
because we'll be doing metadata updates as part of the wal completion, but
it's not a big step from where we are now, and I think the performance
benefit will be worth it.
May I have some example how it's supposed to work please?
At a high level,
1- prepare kv commit. it includes a wal_op_t that describes a
read/modify/write of a csum block within some existing blob.
2- commit the kv txn
3- do the wal read/modify/write
4- calculate new csum
5- in the wal 'cleanup' transaction (which removes the wal event),
also update blob csum_data in the onode or bnode.
In practice, I think we want to pass TransContext down from _wal_apply
into _do_wal_op, and put modified onode or bnode in the txc dirty list.
(We'll need to clear it after the current _txc_finalize call so that it
doesn't have the first phase's dirty stuff still there.) Then, in
_kv_sync_thread, where the
// cleanup sync wal keys
stuff is, we probably want to have a helper that captures the wal event
removal *and* any other stuff we need to do.. like update onodes and
bnodes. It'll look similar to _txc_finalize, I think.
Then the main piece is how to modify the bluestore_wal_op_t to describe
which blob metadata we're modifying and how to do the
whole read/modify/write operation. I think
- we need to bundle the csum data for anything we read. we can probably
just put a blob_t in here, since it includes the extents and csum metadata
all together.
- we need to describe where the blob exists (which onode or bnode owns
it, and what its id is) so that do_wal_op can find it and update it.
* we might want to optimize the normal path so that we can use the
in-memory copy without doing a lookup
It probably means a mostly rewritten wal_op_t type. I think the ops we
need to capture are
- overwrite
- read / modify / write (e.g., partial blocks)
- read / modify / write (and update csum metadata)
- read / modify / compress / write (and update csum metadata)
- read / write elsewhere (e.g., the current copy op, used for cow)
Since compression is thrown in there, we probably need to be able to
allocate in the do_wal_op path too. I think that'll be okay... it's
making the wal finalize kv look more and more like the current
txc_finalize. That probably means if we're careful we can use the same
code for both?
I took a stab at a revised wal_op_t here:
https://github.com/liewegas/ceph/blob/wip-bluestore-write/src/os/bluestore/bluestore_types.h#L595-L605
This is enough to implement the basic wal overwrite case here:
https://github.com/liewegas/ceph/blob/wip-bluestore-write/src/os/bluestore/BlueStore.cc#L5522-L5578
It's overkill for that, but something like this ought to be sufficiently
general to express the more complicated wal (and compaction/gc/cleanup)
operations, where we are reading bits of data from lots of different
previous blobs, verifying checksums, and then assembling the results into
a new buffer that gets written somewhere else. The read_extent_map and
write_map offsets are logical offsets in a buffer we assemble and then
write to b_off~b_len in the specific blob. I didn't get to the _do_wal_op
part that actually does it, but it would do the final write, csum
calculation, and metadata update. Probably... the allocation would happen
then too, if the specified blob didn't already have pextents. Tha way
we can do compression at that stage as well?
What do you think?
sage
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