DM-integrity questions, a storage-stack plan with request for critique/feedback

Hello, all.



Questions...

* Meta-level question: is there a Web-based discussion forum for
DM-integrity?  I searched for one but did not find one -- I found only
the email address "[email protected]".

* What is/are the difference[s] between "direct" and "bitmap" in DM-integrity?

* Does "bitmap mode" in DM-integrity write the bitmap to the
underlying _storage_ while the system is running [as opposed to e.g.
only at {normal-shutdown}-time]?  My concern here is that what
_should_ be large high-throughput streaming writes could become
head-seeking hard-drive-destroying events when some/all of the
underlying storage is on "spinning rust".



For background/critique/feedback...

* I _don`t_ want to use DM-integrity _journalling_, because I plan to
use a journaling _filesystem_ at the top of the relevant stack, and
because -- for the currently-planned application, please see my RAID5
construction plan below -- getting only one-_quarter_ of the
underlying throughput [one quarter at _best_, i.e. _very_-naively
assuming no/zero _seeking_ overhead _despite_ planning to use spinning
rust for the primary storage] in exchange for greatly-enhanced data
safety in the event of a corner-case
{crash,_reset,_or_power-off}_during_a_write is _not_ an acceptable
trade-off IMO.


* I currently plan to use a stack that will look at least something like this:

  [the top level] a modern checksumming filesystem [probably BtrFS --
almost-certainly _not_ ZFS, due to the issues inherent to/with using
out-of-Linus`s-kernel-tree filesystems directly -- as opposed to e.g.
in a virtual machine or via NFS -- on a Linux box]

  Bcache [to try to keep the filesystem metadata on a mirrored pair of
SSDs rather than allowing metadata to be stored only on the spinning
rust]

     Bcache-wise _cache_ storage
     ----------------------------------------
     sub-layer 1: DM-integrity, so that if something in the cache
sub-stack goes wrong in a detectable way, the in-kernel Bcache _might_
[if it is "smart enough"] "realize" that it should read the relevant
block[s] from the primary storage instead, as a fallback

         sub-layer 2: Linux MD RAID1, i.e. mirroring
             sub-layer 3 [the bottom of this cache sub-stack]: two SATA SSDs


     Bcache-wise _primary_ storage
     ------------------------------------------

      Linux MD RAID5
      ----------------------
      * Using the RAID5-specific in-kernel cache code so as to both
alleviate write-hole concerns _and_ -- at least if choosing to use
write-_back_ mode -- increase performance especially in the event of
the next-up layer performing lots of _non_-{full stripe} writes

       * probably going to use this in write-_back_ mode, for
performance, if at all...  but I welcome arguments for write-_through_


        RAID5-wise _cache_ storage
        ---------------------------------------
         sub-layer 1: Linux MD RAID1, i.e. mirroring
             sub-layer 2 [the bottom of this cache sub-stack]: two
ultra-high-throughput writes-are-fine-with-me-I-am-not-an-SSD SCSI
hard drives

        RAID5-wise _primary_ storage
        -----------------------------------------
         DM-integrity to detect silent corruption in
assumed-{untrustworthy-by-design} consumer-grade hard drives

            [at the bottom] raw partitions on >2 consumer-grade SATA hard drives



Please poke holes -- "poking" with logic and detailed explanation[s],
of course ;-) -- in my above "design", and explain to me why I am
wrong/crazy/both.  ;-)



I am also planning to build a
reliability-comes-first-and-performance-comes-second-or-third-or-never
RAID1 [RAID1 for use with a _filesystem_ directly on top of it, i.e.
_not_ RAID1 as a "subroutine" of a larger block stack (as above)] --
any advice on using DM-integrity [or telling me e.g. "don`t do that,
and here`s why not" ;-)] in _this_ context is _also_ welcome.



Sincerely,

Abe
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