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
