(1) ZetaScale uses a B+-tree internally rather than an LSM tree (levelDB/RocksDB). LSM trees experience exponential increase in write amplification (cost of an insert) as the amount of data under management increases. B+tree write-amplification is nearly constant independent of the size of data under management. As the KV database gets larger (Since newStore is effectively moving the per-file inode into the kv data base. Don't forget checksums that Sage want's to add :)) this performance delta swamps all others.
(2) Having a KV and a file-system causes a double lookup. This costs CPU time and disk accesses to page in data structure indexes, metadata efficiency decreases.
You can't avoid (2) as long as you're using a file system.
Yes an LSM tree performs better on HDD than does a B-tree, which is a good argument for keeping the KV module pluggable.
Allen Samuels
Software Architect, Fellow, Systems and Software Solutions
2880 Junction Avenue, San Jose, CA 95134
T: +1 408 801 7030| M: +1 408 780 6416
allen.samuels@xxxxxxxxxxx
-----Original Message-----
From: ceph-devel-owner@xxxxxxxxxxxxxxx [mailto:ceph-devel-owner@xxxxxxxxxxxxxxx] On Behalf Of Ric Wheeler
Sent: Tuesday, October 20, 2015 11:32 AM
To: Sage Weil <sweil@xxxxxxxxxx>; ceph-devel@xxxxxxxxxxxxxxx
Subject: Re: newstore direction
On 10/19/2015 03:49 PM, Sage Weil wrote:
The current design is based on two simple ideas:
1) a key/value interface is better way to manage all of our internal
metadata (object metadata, attrs, layout, collection membership,
write-ahead logging, overlay data, etc.)
2) a file system is well suited for storage object data (as files).
So far 1 is working out well, but I'm questioning the wisdom of #2. A
few
things:
- We currently write the data to the file, fsync, then commit the kv
transaction. That's at least 3 IOs: one for the data, one for the fs
journal, one for the kv txn to commit (at least once my rocksdb
changes land... the kv commit is currently 2-3). So two people are
managing metadata, here: the fs managing the file metadata (with its
own
journal) and the kv backend (with its journal).
If all of the fsync()'s fall into the same backing file system, are you sure that each fsync() takes the same time? Depending on the local FS implementation of course, but the order of issuing those fsync()'s can effectively make some of them no-ops.
- On read we have to open files by name, which means traversing the
fs namespace. Newstore tries to keep it as flat and simple as
possible, but at a minimum it is a couple btree lookups. We'd love to
use open by handle (which would reduce this to 1 btree traversal), but
running the daemon as ceph and not root makes that hard...
This seems like a a pretty low hurdle to overcome.
- ...and file systems insist on updating mtime on writes, even when
it is a overwrite with no allocation changes. (We don't care about
mtime.) O_NOCMTIME patches exist but it is hard to get these past the
kernel brainfreeze.
Are you using O_DIRECT? Seems like there should be some enterprisey database tricks that we can use here.
- XFS is (probably) never going going to give us data checksums,
which we want desperately.
What is the goal of having the file system do the checksums? How strong do they need to be and what size are the chunks?
If you update this on each IO, this will certainly generate more IO (each write will possibly generate at least one other write to update that new checksum).
But what's the alternative? My thought is to just bite the bullet and
consume a raw block device directly. Write an allocator, hopefully
keep it pretty simple, and manage it in kv store along with all of our
other metadata.
The big problem with consuming block devices directly is that you ultimately end up recreating most of the features that you had in the file system. Even enterprise databases like Oracle and DB2 have been migrating away from running on raw block devices in favor of file systems over time. In effect, you are looking at making a simple on disk file system which is always easier to start than it is to get back to a stable, production ready state.
I think that it might be quicker and more maintainable to spend some time working with the local file system people (XFS or other) to see if we can jointly address the concerns you have.
Wins:
- 2 IOs for most: one to write the data to unused space in the block
device, one to commit our transaction (vs 4+ before). For overwrites,
we'd have one io to do our write-ahead log (kv journal), then do the
overwrite async (vs 4+ before).
- No concern about mtime getting in the way
- Faster reads (no fs lookup)
- Similarly sized metadata for most objects. If we assume most
objects are not fragmented, then the metadata to store the block
offsets is about the same size as the metadata to store the filenames we have now.
Problems:
- We have to size the kv backend storage (probably still an XFS
partition) vs the block storage. Maybe we do this anyway (put
metadata on
SSD!) so it won't matter. But what happens when we are storing gobs
of rgw index data or cephfs metadata? Suddenly we are pulling storage
out of a different pool and those aren't currently fungible.
- We have to write and maintain an allocator. I'm still optimistic
this can be reasonbly simple, especially for the flash case (where
fragmentation isn't such an issue as long as our blocks are reasonbly
sized). For disk we may beed to be moderately clever.
- We'll need a fsck to ensure our internal metadata is consistent.
The good news is it'll just need to validate what we have stored in
the kv store.
Other thoughts:
- We might want to consider whether dm-thin or bcache or other block
layers might help us with elasticity of file vs block areas.
- Rocksdb can push colder data to a second directory, so we could
have a fast ssd primary area (for wal and most metadata) and a second
hdd directory for stuff it has to push off. Then have a conservative
amount of file space on the hdd. If our block fills up, use the
existing file mechanism to put data there too. (But then we have to
maintain both the current kv + file approach and not go all-in on kv +
block.)
Thoughts?
sage
--
I really hate the idea of making a new file system type (even if we call it a raw block store!).
In addition to the technical hurdles, there are also production worries like how long will it take for distros to pick up formal support? How do we test it properly?
Regards,
Ric
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