On 13 February 2013 08:56, Kent Overstreet <koverstreet@xxxxxxxxxx> wrote: > On Tue, Feb 12, 2013 at 09:41:20PM +0100, Roy Sigurd Karlsbakk wrote: >> Wouldn't it be better to allow the sysadmin to determine the safety? > > The sysadmin can override, but - say you're using bcache for your root > filesystem and after a reboot the SSD doesn't come up, for whatever > reason. > > How's the sysadmin supposed to know for sure whether there was dirty > data in the cache? There's no way to reliably track that without bcache > tracking it itself in the backing device's superblock. > > Say there wasn't any dirty data in the cache, so you can safely run > without the cache device - so you do, so you can boot up and use your > machine. > > Then later you figure out what's wrong with the SSD (cable got > unplugged?), so you reenable caching. > > But the cache is now inconsistent - you _cannot_ use that cached data. > With the backing device superblock, bcache can trivially note that the > cache is out of sync and make sure that that cached data isn't used. > > If we didn't have that, the sysadmin would have to make _sure_ to use > the right flag when reattaching to specify cached data shouldn't be > used, otherwise he just corrupted all his data. Looking through the enhanceio repo it seems possible to do it without a superblock by utilizing customized udev rules. Said rules prevent the backing device from being using to mount the root filesystem before the cache is attached. > > >> Also, if running in writethrough, why shouldn't an SSD be allowed to be added in realtime? > > If you're running in writethrough, you'd already be caching... not sure > what you mean? > >> All of this works well with systems like ZFS. I really don't see a reason for a filesystem being created to allow caching > > ZFS is a filesystem that also does caching, bcache isn't a filesystem. > Did you get something backwards...? > -- > To unsubscribe from this list: send the line "unsubscribe linux-bcache" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- CTO | Orion Virtualisation Solutions | www.orionvm.com.au Phone: 1300 56 99 52 | Mobile: 0428 754 846 -- To unsubscribe from this list: send the line "unsubscribe linux-bcache" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
