On Mon 16-06-08 12:55:24, Andreas Dilger wrote: > On Jun 16, 2008 17:05 +0200, Jan Kara wrote: > > First, I'd like to see some short comment on what semantics > > delalloc,data=ordered is going to have. At least I can imagine at least > > two sensible approaches: > > 1) All we guarantee is that user is not going to see uninitialized data. > > We send writes to disk (and allocate blocks) whenever it fits our needs > > (usually when pdflush finds them). > > 2) We guarantee that when transaction commits, your data is on disk - > > i.e., we allocate actual blocks on transaction commit. > > > > Both these possibilities have their pros and cons. Most importantly, > > 1) gives better disk layout while 2) gives higher consistency > > guarantees. Note that with 1), it can under some circumstances happen, > > that after a crash you see block 1 and 3 of your 3-block-write on disk, > > while block 2 is still a hole. 1) is easy to implement (you mostly did > > it below), 2) is harder. I think there should be broader consensus on > > what the semantics should be (changed subject to catch more attention ;). > > IMHO, the semantic should be (1) and not (2). Applications don't understand > "when the transaction commits" so it doesn't provide any useful guarantee > to userspace, and if they actually need the data on disk (e.g. MTA) then > they need to call fsync to ensure this. Well, in principle I agree with you. But let's take example of firefox / konqueror. They call fsync() on each modification to their "browsing history" file. And when I asked one Konqueror developer why do they do such a stupid thing, he told me that XFS used to zero-out portions of the file on crash (because of delalloc) and this broke the browser in funny ways. Now the point I wanted to make is that if the filesystem behaves "reasonably" apps often do not need to call fsync (Konquerror is fine with how current ext3 in data=ordered mode behaves without using fsync, I'm not sure if it would be fine with delalloc,data=ordered of ext4). So I'm not really decided whether the additional performance is worth the decreased data consistency guarantees... > While I agree it is theoretically possible to have the "hole in data > where there shouldn't be one" scenario, in real life these blocks would be > allocated together by delalloc+mballoc and this situation should not happen. > > As for "sync with heavy IO causing slowness" problem of Firefox, I think > that delalloc will help this noticably, but I agree we can still get into > cases where a lot of dirty data was just allocated and now needs to be > flushed to disk to commit the transaction. > > In the short term I don't think this can be completely fixed, but in the > long term I think it can be fixed by having mballoc do "reservations" of > space on disk, in which the dirty pages can be written. Only after the > data is on disk does the "reservation" turn into an "allocation" in the > journal (i.e. filesystem buffers added to transaction and modified). > At that point a sync operation only has to write out the journal blocks, > because all of the data is on disk already. Interesting idea, this seems like a viable way to fix the problem. > I don't think it is a huge difference from what we have today, but I > also don't think it should be in the first implementation. We would > need to split up handling of the in-memory block bitmaps so that only > the in-memory ones are updated first, then the on-disk bitmaps are > later marked in use in a transaction after the data blocks are on disk. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
