path: root/Documentation/filesystems/dax.txt
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diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt
index 7bde64014a89..ce4587d257d2 100644
--- a/Documentation/filesystems/dax.txt
+++ b/Documentation/filesystems/dax.txt
@@ -79,6 +79,38 @@ 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 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. Truncate or hole-punch the part of the file that has a bad-block (at least
+ an entire aligned sector has to be hole-punched, but not necessarily an
+ entire filesystem block).
+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).