HI all.. So this sucks, having to check every single disk image so it wont get corrupted stuff, while it works on other filesystems isn't exactly stellar.
i changed the qcow2 settings per this page: https://pve.proxmox.com/wiki/Performance_Tweaks i went with cache=writeback since it neither uses O_DSYNC nor O_DIRECT semantics. I understand the reasons checksumming might fail and that isn't exactly btrfs fault but i scoured the entire btrfs wiki and couldnt find anything warning about btrfs + qcow2 (or other image types) in the wiki pages. Maybe adding some warning would help unlucky ppl like myself? Also, is is safe to enable compress=lzo , or is it also a no no, im also starting to suspect that discard wasnt the culprit since i have it enabled for more then a year and the only problem i got with corruption was precisely this images? | Paulo Dias | [email protected] Tempora mutantur, nos et mutamur in illis. On Fri, Aug 18, 2017 at 2:59 PM, Liu Bo <[email protected]> wrote: > On Fri, Aug 18, 2017 at 06:23:18PM +0200, Goffredo Baroncelli wrote: >> On 08/18/2017 01:39 AM, Josef Bacik wrote: >> [...] >> > This is happening because the app (the guest OS in this case, we saw this >> > a lot >> > with windows guests) is changing the pages while they are in flight. We >> > calculate the checksum of the page before it's written, so if it changes >> > while >> > in flight we'll end up with a csum mismatch. >> > >> > To fix this change kvm to not use O_DIRECT or set NODATASUM on your qcow2 >> > image. >> > You'll have to re-create the image because NODATASUM won't apply to the >> > already >> > invalid checksums. Thanks, >> >> Hi Josef, >> >> could you elaborate: do you are saying that using O_DIRECT is incompatible >> with DATASUM ? >> > > They're compatible, but applications need to be careful. > > O_DIRECT takes userspace page, and it works like > DIO write > p = get_user_page(); > add p to bio > #btrfs submits this bio > calc_checksum(bio); > submit_bio(); > > There's a chance that page p got changed between calc_checksum() and > submit_bio(), which then causes the mismatch. > > For buffered IO, dirty page cache pages is synchronized with page > fault by page lock and page writeback bit. > > thanks, > -liubo -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
