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The metadata low watermark threshold is set by the kernel. But the kernel depends on userspace to extend the thinpool metadata device when the threshold is crossed. Since the metadata low watermark threshold is not visible to userspace, upon receiving an event, userspace cannot tell that the kernel wants the metadata device extended, instead of some other eventing condition. Making it visible (but not settable) enables userspace to affirmatively know the kernel is asking for a metadata device extension, by comparing metadata_low_watermark against nr_free_blocks_metadata, also reported in status. Current solutions like dmeventd have their own thresholds for extending the data and metadata devices, and both devices are checked against their thresholds on each event. This lessens the value of the kernel-set threshold, since userspace will either extend the metadata device sooner, when receiving another event; or will receive the metadata lowater event and do nothing, if dmeventd's threshold is less than the kernel's. (This second case is dangerous. The metadata lowater event will not be re-sent, so no further event will be generated before the metadata device is out if space, unless some other event causes userspace to recheck its thresholds.) Signed-off-by: Andy Grover <agrover@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com> |
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| .. | ||
| cache-policies.txt | ||
| cache.txt | ||
| delay.txt | ||
| dm-crypt.txt | ||
| dm-flakey.txt | ||
| dm-integrity.txt | ||
| dm-io.txt | ||
| dm-log.txt | ||
| dm-queue-length.txt | ||
| dm-raid.txt | ||
| dm-service-time.txt | ||
| dm-uevent.txt | ||
| dm-zoned.txt | ||
| era.txt | ||
| kcopyd.txt | ||
| linear.txt | ||
| log-writes.txt | ||
| persistent-data.txt | ||
| snapshot.txt | ||
| statistics.txt | ||
| striped.txt | ||
| switch.txt | ||
| thin-provisioning.txt | ||
| unstriped.txt | ||
| verity.txt | ||
| writecache.txt | ||
| zero.txt | ||