On 06/16/2014 03:52 PM, Martin wrote: > On 16/06/14 17:05, Josef Bacik wrote: >> >> On 06/16/2014 03:14 AM, Lennart Poettering wrote: >>> On Mon, 16.06.14 10:17, Russell Coker (russ...@coker.com.au) wrote: >>> >>>>> I am not really following though why this trips up btrfs though. I am >>>>> not sure I understand why this breaks btrfs COW behaviour. I mean, > >>>> I don't believe that fallocate() makes any difference to >>>> fragmentation on >>>> BTRFS. Blocks will be allocated when writes occur so regardless of an >>>> fallocate() call the usage pattern in systemd-journald will cause >>>> fragmentation. >>> >>> journald's write pattern looks something like this: append something to >>> the end, make sure it is written, then update a few offsets stored at >>> the beginning of the file to point to the newly appended data. This is >>> of course not easy to handle for COW file systems. But then again, it's >>> probably not too different from access patterns of other database or >>> database-like engines... > > Even though this appears to be a problem case for btrfs/COW, is there a > more favourable write/access sequence possible that is easily > implemented that is favourable for both ext4-like fs /and/ COW fs? > > Database-like writing is known 'difficult' for filesystems: Can a data > log can be a simpler case? > > >> Was waiting for you to show up before I said anything since most systemd >> related emails always devolve into how evil you are rather than what is >> actually happening. > > Ouch! Hope you two know each other!! :-P :-) > > > [...] >> since we shouldn't be fragmenting this badly. >> >> Like I said what you guys are doing is fine, if btrfs falls on it's face >> then its not your fault. I'd just like an exact idea of when you guys >> are fsync'ing so I can replicate in a smaller way. Thanks, > > Good if COW can be so resilient. I have about 2GBytes of data logging > files and I must defrag those as part of my backups to stop the system > fragmenting to a stop (I use "cp -a" to defrag the files to a new area > and restart the data software logger on that). > > > Random thoughts: > > Would using a second small file just for the mmap-ed pointers help avoid > repeated rewriting of random offsets in the log file causing excessive > fragmentation? > > Align the data writes to 16kByte or 64kByte boundaries/chunks? > > Are mmap-ed files a similar problem to using a swap file and so should > the same "btrfs file swap" code be used for both? > > > Not looked over the code so all random guesses... > > Regards, > Martin > > > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Just a thought, partly inspired by the mention of the swap code, has anyone tried making the file NOCOW and pre-allocating to the max journal size? A similar approach has seemed to help on my systems with generic log files (I keep debug level logs from almost everything, so I end up with very active log files with ridiculous numbers of fragments if I don't pre-allocate and mark them NOCOW). I don't know for certain how BTRFS handles appends to NOCOW files, but I would be willing to bet that it ends up with a new fragment for each filesystem block worth of space allocated.
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