On 08/27/2009 04:46 PM, James Bottomley wrote: > On Thu, 2009-08-27 at 12:49 +0300, Boaz Harrosh wrote: >> On 08/27/2009 09:34 AM, Martin K. Petersen wrote: >>>>>>>> "Boaz" == Boaz Harrosh <bharrosh@xxxxxxxxxxx> writes: >>> >>> Boaz> I know that we also have the above problem with iscsi and >>> Boaz> data-digest such that when we come to sign the data it might >>> Boaz> change on us before the target receives it. >>> >>> Yep, I have the same problem. I talked to Andrew Morton a couple of >>> months ago and he said that modifying pages in flight is "a feature" as >>> far as ext[234] is concerned. >>> >> >> As you might know, I have a filesystem copied from the ext2 code base. >> I'm experimenting with altering the behavior so that pages written to >> while been IOed will page fault, then sleep, until IO is done. >> Clearly this is a good "feature" until such systems like mirror or signed- >> data that are forced to reallocate-copy all IO do to the 2% optimization >> that thing gives you. > > What about reads to the page? If you allow them, you get the situation > where something signals a write intent, tries to write and gets put into > wait, then the readers get the old data still. > Is there any guaranty between a parallel write and read about what's first? But I think in my case the reads will also page-fault so I'm not sure yet. Thanks for asking that's a good question that should be taken into consideration. >> At the final outcome I hope for a VFS support on a flip of a flag or >> something. So under laying device can turn that "feature" off when it >> means grate performance gains in it's operations. >> >> If any one has thought about that problem, and as some preliminary strategies, >> please I'm all hears. I've just started on this subject and currently I do not >> have a clue. > > The correct way to handle this is simply to dump the page being written. > It's dirty and was updated after the last write attempt, so it gets > re-written out. It costs nothing and it's incredibly fast. > This is not an option on a mirror system, and the performance gain/lose is dependent on the round trip speed. If for every digest error I have an error recovery cycle, delays, and stalls. Then no it is not better. Not to mention some iscsi-targets that reset and the all session must be re-established. > What you likely want is a way of telling that the page got re-written so > you don't need to print out scary warning messages about parity > problems. > Maybe that is a start. I guess I could signal a fast abort for these. What would be the cost for this knowledge. I guess O(sglist-size) right? loop on all pages and check? Anything better we can do? > James > > Thanks Boaz -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
