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zwol
Asked: 2021-07-03 04:55:20 +0800 CST

What does the Cpy%Sync column of lvs output mean for a cache LV?

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I have a host with a single cache LV (~800GB SSD in front of a multi-TB RAID array; write-through mode). I ran a routine scrub on the RAID last night (lvchange --syncaction check). Ever since that completed, the system has been running at 100% I/O capacity; iostat tells me it's reading data from the SSD and writing to the RAID.

The Cpy%Sync column of lvs output for the cache LV (not the RAID) started out at 98% or so before the scrub, which was worrisome in itself, but now it is steadily going down, having currently reached about 70%.

I thought Cpy%Sync for a cache LV indicated the amount of data on the cache that also existed on the backing store, so in write-through mode it should always be 100%. What does it actually mean and what is the system doing that's making it go down steadily?

There is nothing helpful in dmesg.

lvm dm-cache

1 Answers

  1. According to the lvmraid man pages [1]:

    Automatic synchronization happens when a RAID LV is activated, but it is usually partial because the bitmaps reduce the areas that are checked. A full sync becomes necessary when devices in the RAID LV are replaced.

    The synchronization status of a RAID LV is reported by the following command, where "Cpy%Sync" = "100%" means sync is complete:

    lvs -a -o name,sync_percent

    Another excerpt:

    When integrity is added to an LV, the kernel needs to initialize the integrity metadata (checksums) for all blocks in the LV. The data corruption checking performed by dm-integrity will only operate on areas of the LV that are already initialized. The progress of integrity initialization is reported by the "syncpercent" LV reporting field (and under the Cpy%Sync lvs column.)

    However, another source claiming a similar problem [2] mentions the following in regards to the disk activity:

    Cache is always flushing it's content to disk - so if you have the 'writeback' variant and there were some unwritten data which sits only in cache, they are automatically pushed to origin (cached) device - which is likely your disk activity.

    1. https://man7.org/linux/man-pages/man7/lvmraid.7.html
    2. https://listman.redhat.com/archives/linux-lvm/2015-June/msg00001.html
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