On Mon, Nov 18, 2019 at 08:05:22AM +0000, Michael Chang wrote: > On Thu, Oct 24, 2019 at 05:21:04PM +0200, Daniel Kiper wrote: > > On Thu, Oct 24, 2019 at 04:43:58AM +0000, Michael Chang wrote: > > > On Wed, Oct 23, 2019 at 12:48:23PM +0200, Daniel Kiper wrote: > > > > On Wed, Oct 16, 2019 at 06:15:30AM +0000, Michael Chang wrote: > > > > > The lvm cache logical volume is the logical volume consisting of the > > > > > original and the cache pool logical volume. The original is usually > > > > > on a > > > > > larger and slower storage device while the cache pool is on a smaller > > > > > and faster one. The performance of the original volume can be improved > > > > > by storing the frequently used data on the cache pool to utilize the > > > > > greater performance of faster device. > > > > > > > > > > The default cache mode "writethrough" ensures that any data written > > > > > will > > > > > be stored both in the cache and on the origin LV, therefore grub can > > > > > go > > > > > straight to read the original lv as no data loss is guarenteed. > > > > > > > > > > The second cache mode is "writeback", which delays writing from the > > > > > cache pool back to the origin LV to have increased performance. The > > > > > drawback is potential data loss if losing the associated cache device. > > > > > > > > > > During the boot time grub reads the LVM offline i.e. LVM volumes are > > > > > not > > > > > activated and mounted, IMHO it should be fine to read directly from > > > > > original lv since all cached data should have been flushed back in the > > > > > process of taking it offline. > > > > > > > > Is it possible to enforce all GRUB writes to the original device instead > > > > of cache during installation process? > > > > > > I couldn't give concrete answer whether it is possible, but it sounds to > > > me not good idea because in general bypassing the cache could > > > potentially break the consistency of data and the data being cached. > > > > > > Perhaps some system calls could help to sync the data out of the lvm > > > cache to the original LV during installation process. It seems fsync > > > does it for us, but I didn't have good idea either if it is enough. > > > > May I ask you to investigate that and if it is needed add required > > install code. I mean fsync(), etc. > > As far as what I learned, the fsync did not write back dirty cache to > the orginal device, instead it would update associated cache metadata to > complete the write transaction with the cache device and then having the > write cache up to date. Without the operation, metadata is commited > every second and data would be completely lost/evaporated if power > failure happened in between. > > To write back dirty cache, as lvm cache did not support dirty cache > flush per block range, there'no way to do it for file. Instead the > "cleaner" policy is implemented and can be used to write back "all" > dirty blocks in a cache, which effectively drain all dirty cache > gradually to attain and last in the "clean" state, which can be useful > for shrinking or decommissioning a cache. The result and effect is not > what we are looking for here. > > In conclusion, it seems no way to enforce all grub writes to the > original device. That means grub may suffer from power failure that > leads to dirty cache not being flushed and is inept reading data flagged > as dirty in cache. But even though, as long as such case is only > subjected to writeback mode that is generally difficult to deal with > data lost whenever accident comes up, I'd still like to propose my > (relatively simple) patch and treat reading dirty data in cache as an > enhancement to be more resilience to potential data loss.
I would like to see more or less this description in the commit message. And I think that you should add similar paragraph to the GRUB manual too. Daniel _______________________________________________ Grub-devel mailing list Grub-devel@gnu.org https://lists.gnu.org/mailman/listinfo/grub-devel
