efc412b645
The spa_config_write() function relies on the classic method of making sure updates to the /etc/zfs/zpool.cache file are atomic. It writes out a temporary version of the file and then uses vn_rename() to switch it in to place. This way there can never exist a partial version of the file, it's all or nothing. Conceptually this is a good strategy and it makes good sense for platforms where it's easy to do a rename within the kernel. Unfortunately, Linux is not one of those platforms. Even doing basic I/O to a file system from within the kernel is strongly discouraged. In order to support this at all the vn_rename() implementation ends up being complex and fragile. So fragile that recent Linux 4.2 changes have broken it. While it is possible to update vn_rename() to work with the latest kernels a better long term strategy is to stop using vn_rename() entirely. Then all this complex, fragile code can be removed. Achieving this is straight forward because config_write() is the only consumer of vn_rename(). This patch reworks spa_config_write() to update the cache file in place. The file will be truncated, written out, and then synced to disk. If an error is encountered the file will be unlinked leaving the system in a consistent state. This does expose a tiny tiny tiny window where a system could crash at exactly the wrong moment could leave a partially written cache file. However, this is highly unlikely because the cache file is 1) infrequently updated, 2) only a few kilobytes in size, and 3) written with a single vn_rdwr() call. If this were to somehow happen it poses no risk to pool. Simply removing the cache file will allow the pool to be imported cleanly. Going forward this will be even less of an issue as we intend to disable the use of a cache file by default. Bottom line not using vn_rename() allows us to make ZoL more robust against upstream kernel changes. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #3653 |
||
|---|---|---|
| .. | ||
| avl | ||
| nvpair | ||
| unicode | ||
| zcommon | ||
| zfs | ||
| zpios | ||
| .gitignore | ||
| Makefile.in | ||