On Tue, Feb 12, 2019 at 5:10 AM Hector Martin <hector@xxxxxxxxxxxxxx> wrote:
On 12/02/2019 06:01, Gregory Farnum wrote:
> Right. Truncates and renames require sending messages to the MDS, and
> the MDS committing to RADOS (aka its disk) the change in status, before
> they can be completed. Creating new files will generally use a
> preallocated inode so it's just a network round-trip to the MDS.
I see. Is there a fundamental reason why these kinds of metadata
operations cannot be buffered in the client, or is this just the current
way they're implemented?
It's pretty fundamental, at least to the consistency guarantees we hold ourselves to. What happens if the client has buffered an update like that, performs writes to the data with those updates in mind, and then fails before they're flushed to the MDS? A local FS doesn't need to worry about a different node having a different lifetime, and can control the write order of its metadata and data updates on belated flush a lot more precisely than we can. :(
-Greg
e.g. on a local FS these kinds of writes can just stick around in the
block cache unflushed. And of course for CephFS I assume file extension
also requires updating the file size in the MDS, yet that doesn't block
while truncation does.
> Going back to your first email, if you do an overwrite that is confined
> to a single stripe unit in RADOS (by default, a stripe unit is the size
> of your objects which is 4MB and it's aligned from 0), it is guaranteed
> to be atomic. CephFS can only tear writes across objects, and only if
> your client fails before the data has been flushed.
Great! I've implemented this in a backwards-compatible way, so that gets
rid of this bottleneck. It's just a 128-byte flag file (formerly
variable length, now I just pad it to the full 128 bytes and rewrite it
in-place). This is good information to know for optimizing things :-)
--
Hector Martin (hector@xxxxxxxxxxxxxx)
Public Key: https://mrcn.st/pub
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