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. > > 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. I'm not sure that's true; filling in holes is not that uncommon. But, I'm not sure that it actually leads to a problem, as the metadata gets "created" for the hole-fill-in only when the block actually gets allocated right? -Eric -- 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
