HAMMER update 23-Mar-08
dillon at apollo.backplane.com
Mon Mar 24 20:10:12 PDT 2008
:Wouldn't making timestamp queries (at least from userland) enforce a
:sync on the volume in question be useful here?
: Thomas E. Spanjaard
: tgen at netphreax.net
Making the 'hammer now' command do a sync() is a good idea. I will
make that change right now so it doesn't get lost.
Here's a general overview of the issues involved with having historical
access to the filesystem:
Recording the timestamps in the in-memory cache, for a finer-grained
snapshot capability, is doable but has its own issues. Here's an
open() create file
write() append 4K (file size now 4K)
write() append 4K (file size now 8K)
write() append 4K (file size now 12K)
write() append 4K (file size now 16K)
Now NONE of this has gone to disk yet, it's entirely in the in-memory
cache. The inode is in the in-memory cache. The data is stored in
the buffer cache. Even the directory entry for the file that we
just created is still in the in-memory cache (HAMMER caches the raw
records it intends to commit later on).
If I wanted to be able to acquire a timestamp between each write and
'see' a snapshot of the file as of any point in the above sequence,
then every write would also have to allocate a copy of the inode
(because it changes size on each write).
The data has the same problem though with a slightly different example.
Lets say each write() was a seek-write, overwriting the previous data.
Now with every write() I would have to allocate a copy of the data
being overwritten. This is complicated by the fact that the buffer
cache has no clue about 'historical' accesses, so I would not be able
to use the buffer cache to cache the data.
There's also another problem and that is with the efficiency of the
topology on-disk. Even if I maintained all the copies of the inode
and all the copies of the data in-memory, I would still have to sync
all those copies to disk in order for things to remain historically
coherent (whether it be in-cache or on-disk). This would result in
hundreds or even thousands of copies of the inode on-disk, not to
mention potentially many copies of the data.
I just don't want to do that right now, at least not as a default. A
lot of performance would be lost. Hence a sync() is needed if
you want to create a demark which you can accurately snapshot.
Here's a quick synopsis of how the cache would operate in a clustered
In order to properly integrate with in-memory caches, a wider cache
coherency infrastructure is needed between machines such that
modifications made on one machine proactively invalidate those
protions of the cache(s) on other machines. At the same time, any
'dirty' cache data, for example when a file is created or written to,
must lock the cache space in question on all other machines.
The cache space in this case is not just the file data, but also the
related namespaces (for creations, deletions, and renames). Attempts
to access locked spaces from other machines in the cluster would have
to force a flush to the filesystem backing store and lower the cache
states for the effected information on the original machine from dirty
It will be easiest to integrate the cache coherency information into
the buffer cache and namecache themselves. Once a machine has dirtied
an in-memory cache element... for example part of the namespace when
creating a file or chunks of data written within a file, that machine
must have a free hand to make further modifications to the cache spaces
involved without further interaction with other machines.
Now, if you think of those two major elements you can see that they
actually fit together quite well. If I were to attempt to maintain
transactional coherency on a per-system-call basis then the cache
granularity between machines would have to be much, much smaller then
our current in-memory caching elements provide. That would become
a really nasty coding problem. So I don't even want to begin to
complate transactional coherency at a finer-grain then sync() or
fsync() until long after we actually have clustering working.
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