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Re: [dm-devel] NetBSD libdevmapper port



On Fri, 13 Jun 2008, Alasdair G Kergon wrote:

On Fri, Jun 13, 2008 at 12:51:48PM -0400, Mikulas Patocka wrote:
This suspend thing was a big misdesign and if you are writing it from
scratch, try to avoid it.

Despite how things might seem, we gained some substantial
simplifications by doing things this way and I reject the term
'misdesign':-)  If writing this from scratch under similar time
constraints, we would still handle this a similar way.

If you sum up the time that you spent looking for the deadlocks (for example that one when writing to raw device while suspended updated inode mtime field and the kernel tried to write that inode to the suspended root device), thinking how to do proper memory and stack preallocation and locking, how to allocate structure for ioctl arguments (that trick with PF_MEMALLOC, still not bug-free, but at least works in typical scenarios) + plus the time still needed to fix things that are still broken (that GFP_KERNEL allocation in direct IO path, PF_MEMALLOC allocation in ioctl parameter copying) --- and compare this time to the hypothetical time how long it would take to write IOCTL call that performs few operations in a batch and never returns to userspace with suspended device --- which one do you think would win? I think the second solution would be written faster and with fewer bugs.

BTW. is there any need to update on-disk metadata while suspended? What would happen if we first updated metadata and then did suspend+resume in just one syscall? For linear or snapshots, there should be no problem, I'm interested if there's a dm target where this could produce a race condition.

Mikulas

But this suspend/resume mechanism was never intended to last as long as
it has - it was meant to be a stepping stone to a more-sophisticated
transaction mechanism that we have still not found time to develop,
mostly because this existing mechanism is "good enough" most of the
time.

Alasdair
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agk redhat com

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