This just provides information of the basic paths that can be used to deal with (i.e. clear) media errors from the file system point-of-view. Cc: Dave Chinner <david@xxxxxxxxxxxxx> Signed-off-by: Vishal Verma <vishal.l.verma@xxxxxxxxx> --- While this isn't a design document for new mechanisms for adding error recovery/redundancy at the block/fs layers, this attempts to explain the bare essentials required for anything operating above the pmem block driver in the stack. Documentation/filesystems/dax.txt | 34 ++++++++++++++++++++++++++++++++++ 1 file changed, 34 insertions(+) diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt index 7bde640..71cd8fa 100644 --- a/Documentation/filesystems/dax.txt +++ b/Documentation/filesystems/dax.txt @@ -79,6 +79,40 @@ These filesystems may be used for inspiration: - ext4: the fourth extended filesystem, see Documentation/filesystems/ext4.txt +Handling Media Errors +--------------------- + +The libnvdimm subsystem stores a record of known media error locations for +each pmem block device (in gendisk->badblocks). If we fault at such location, +or one with a latent error not yet discovered, the application can expect +to receive a SIGBUS. Libnvdimm also allows clearing of these errors by simply +writing the affected sectors (through the pmem driver, and if the underlying +NVDIMM supports the clear_poison DSM defined by ACPI). + +Since DAX IO normally doesn't go through the driver/bio path, applications or +sysadmins have an option to restore the lost data from a prior backup/inbuilt +redundancy in the following two ways: + +1. Delete the affected file, and restore from a backup (sysadmin route): + This will free the file system blocks that were being used by the file, + and the next time they're allocated, they will be zeroed first, which + happens through the driver, and will clear bad sectors. + +2. Open the file with O_DIRECT, and restore a sector's worth of data at the + bad location (application route): + We allow O_DIRECT writes to go through the normal O_DIRECT path that sends + bios down through the driver. If an application is able to restore its own + data, it can use this path to clear errors. + +These are the two basic paths that allow DAX filesystems to continue operating +in the presence of media errors. More robust error recovery mechanisms can be +built on top of this in the future, for example, involving redundancy/mirroring +provided at the block layer through DM, or additionally, at the filesystem +level. These would have to rely on the above two tenets, that error clearing +can happen either by sending an IO through the driver, or zeroing (also through +the driver). + + Shortcomings ------------ -- 2.5.5 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
