No description
0454b04453
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 be 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, hence 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. It is also not much helpful to the situation by adding fsync calls to the install code. The fsync did not force to write back dirty cache to the original device and rather it would update associated cache metadata to complete the write transaction with the cache device. IOW the writes to cached blocks still go only to the cache device. 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. On the other hand 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, as it seems no way to enforce file writes to the original device, grub may suffer from power failure as it cannot assemble the cache device and read the dirty data from it. However since the case is only applicable to writeback mode which is sensitive to data lost in nature, I'd still like to propose my (relatively simple) patch and treat reading dirty cache as improvement. Signed-off-by: Michael Chang <mchang@suse.com> Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com> |
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| asm-tests | ||
| conf | ||
| docs | ||
| grub-core | ||
| include | ||
| po | ||
| tests | ||
| themes/starfield | ||
| unicode | ||
| util | ||
| .gitattributes | ||
| .gitignore | ||
| .travis.yml | ||
| acinclude.m4 | ||
| AUTHORS | ||
| autogen.sh | ||
| bootstrap | ||
| bootstrap.conf | ||
| BUGS | ||
| ChangeLog-2015 | ||
| config.h.in | ||
| configure.ac | ||
| COPYING | ||
| coreboot.cfg | ||
| geninit.sh | ||
| gentpl.py | ||
| INSTALL | ||
| linguas.sh | ||
| Makefile.am | ||
| Makefile.util.def | ||
| NEWS | ||
| README | ||
| THANKS | ||
| TODO | ||
This is GRUB 2, the second version of the GRand Unified Bootloader. GRUB 2 is rewritten from scratch to make GNU GRUB cleaner, safer, more robust, more powerful, and more portable. See the file NEWS for a description of recent changes to GRUB 2. See the file INSTALL for instructions on how to build and install the GRUB 2 data and program files. Please visit the official web page of GRUB 2, for more information. The URL is <http://www.gnu.org/software/grub/grub.html>. More extensive documentation is available in the Info manual, accessible using 'info grub' after building and installing GRUB 2. There are a number of important user-visible differences from the first version of GRUB, now known as GRUB Legacy. For a summary, please see: info grub Introduction 'Changes from GRUB Legacy'