Re: Feature requests: online backup - defrag - change RAID level

Zygo Blaxell
2019-09-14T04:57:04Z

On Sat, Sep 14, 2019 at 12:42:19AM -0400, Zygo Blaxell wrote:
> On Fri, Sep 13, 2019 at 09:50:38PM -0400, General Zed wrote:
> >
> > Quoting Zygo Blaxell <ce3g8jdj@umail.furryterror.org>:
> >
> > > On Fri, Sep 13, 2019 at 01:05:52AM -0400, General Zed wrote:
> > > >
> > > > Quoting Zygo Blaxell <ce3g8jdj@umail.furryterror.org>:
> > > >
> > > > > On Thu, Sep 12, 2019 at 08:26:04PM -0400, General Zed wrote:
> > > > > >
> > > > > > Quoting Zygo Blaxell <ce3g8jdj@umail.furryterror.org>:
> > > > > >
> > > > > > > Don't forget you have to write new checksum and free space tree pages.
> > > > > > > In the worst case, you'll need about 1GB of new metadata pages
> > > > for each
> > > > > > > 128MB you defrag (though you get to delete 99.5% of them immediately
> > > > > > > after).
> > > > > >
> > > > > > Yes, here we are debating some worst-case scenaraio which is actually
> > > > > > imposible in practice due to various reasons.
> > > > >
> > > > > No, it's quite possible. A log file written slowly on an active
> > > > > filesystem above a few TB will do that accidentally. Every now and then
> > > > > I hit that case. It can take several hours to do a logrotate on spinning
> > > > > arrays because of all the metadata fetches and updates associated with
> > > > > worst-case file delete. Long enough to watch the delete happen, and
> > > > > even follow along in the source code.
> > > > >
> > > > > I guess if I did a proactive defrag every few hours, it might take less
> > > > > time to do the logrotate, but that would mean spreading out all the
> > > > > seeky IO load during the day instead of getting it all done at night.
> > > > > Logrotate does the same job as defrag in this case (replacing a file in
> > > > > thousands of fragments spread across the disk with a few large fragments
> > > > > close together), except logrotate gets better compression.
> > > > >
> > > > > To be more accurate, the example I gave above is the worst case you
> > > > > can expect from normal user workloads. If I throw in some reflinks
> > > > > and snapshots, I can make it arbitrarily worse, until the entire disk
> > > > > is consumed by the metadata update of a single extent defrag.
> > > > >
> > > >
> > > > I can't believe I am considering this case.
> > > >
> > > > So, we have a 1TB log file "ultralog" split into 256 million 4 KB extents
> > > > randomly over the entire disk. We have 512 GB free RAM and 2% free disk
> > > > space. The file needs to be defragmented.
> > > >
> > > > In order to do that, defrag needs to be able to copy-move multiple extents
> > > > in one batch, and update the metadata.
> > > >
> > > > The metadata has a total of at least 256 million entries, each of some size,
> > > > but each one should hold at least a pointer to the extent (8 bytes) and a
> > > > checksum (8 bytes): In reality, it could be that there is a lot of other
> > > > data there per entry.
> > >
> > > It's about 48KB per 4K extent, plus a few hundred bytes on average for each
> > > reference.
> >
> > Sorry, could you be more clear there? An file fragment/extent that holds
> > file data can be any
> > size up to 128 MB. What metadata is there per every file fragment/extent?
> >
> > Because "48 KB per 4 K extent" ... cannot decode what you mean.
>
> An extent has 3 associated records in btrfs, not including its references.
> The first two exist while the extent exists, the third appears after it
> is removed.
>
> - extent tree: location, size of extent, pointers to backref trees.
> Length is around 60 bytes plus the size of the backref pointer list.
>
> - csum tree: location, 1 or more 4-byte csums packed in an array.
> Length of item is number of extent data blocks * 4 bytes plus a
> 168-bit header (ish...csums from adjacent extents may be packed
> using a shared header)
>
> - free space tree: location, size of free space. This appears
> when the extent is deleted. It may be merged with adjacent
> records. Length is maybe 20 bytes?
>
> Each page contains a few hundred items, so if there are a few hundred
> unrelated extents between extents in the log file, each log file extent
> gets its own metadata page in each tree.
>
> > Another question is: what is the average size of metadata extents?
>
> Metadata extents are all 16K.
>
> > > > The metadata is organized as a b-tree. Therefore, nearby nodes should
> > > > contain data of consecutive file extents.
> > >
> > > It's 48KB per item.
> >
> > What's the "item"?
>
> Items are the objects stored in the trees. So one extent item, one csum
> item, and one free space tree item, all tied to the 4K extent from the
> log file.
>
> > > As you remove the original data extents, you will
> > > be touching a 16KB page in three trees for each extent that is removed:
> > > Free space tree, csum tree, and extent tree. This happens after the
> > > merged extent is created. It is part of the cleanup operation that
> > > gets rid of the original 4K extents.
> >
> > Ok, but how big are free space tree and csum tree?
>
> At least 12GB in the worst-case example.

The filesystem where I was hitting the 1GB-metadata issue has 79GB
of metadata. An average of 500-1000 new extents are created on the
filesystem between log file page writes, maybe up to 5000 during peak
activity periods.

> > Also, when moving a file to defragment it, there should still be some
> > locality even in free space tree.
>
> It is only guaranteed in the defrag result, because the defrag result is
> the only thing that is necessarily physically contiguous.
>
> > And the csum tree, it should be ordered similar to free space tree, right?
>
> They are all ordered by extent physical address (same physical blocks,
> same metadata item key).
>
> > > Because the file was written very slowly on a big filesystem, the extents
> > > are scattered pessimally all over the virtual address space, not packed
> > > close together. If there are a few hundred extent allocations between
> > > each log extent, then they will all occupy separate metadata pages.
> >
> > Ok, now you are talking about your pathological case. Let's consider it.
> >
> > Note that there is very little that can be in this case that you are
> > describing. In order to defrag such a file, either the defrag will take many
> > small steps and therefore it will be slow (because each step needs to
> > perform an update to the metadata), or the defrag can do it in one big step
> > and use a huge amount of RAM.
> >
> > So, the best thing to be done in this situation is to allow the user to
> > specify the amount of RAM that defrag is allowed to use, so that the user
> > decides which of the two (slow defrag or lots of RAM) he wants.
> >
> > There is no way around it. There is no better defrag than the one that has
> > ALL information at hand, that one will be the fastest and the best defrag.
> >
> > > When it is time to remove them, each of these pages must be updated.
> > > This can be hit in a number of places in btrfs, including overwrite
> > > and delete.
> > >
> > > There's also 60ish bytes per extent in any subvol trees the file
> > > actually appears in, but you do get locality in that one (the key is
> > > inode and offset, so nothing can get between them and space them apart).
> > > That's 12GB and change (you'll probably completely empty most of the
> > > updated subvol metadata pages, so we can expect maybe 5 pages to remain
> > > including root and interior nodes). I haven't been unlucky enough to
> > > get a "natural" 12GB, but I got over 1GB a few times recently.
> >
> > The thing that I figured out (and I have already written it down in another
> > post) is that the defrag can CHOOSE AT WILL how large update to metadata it
> > wants to perform (within the limit of available RAM). The defrag can select,
> > by itself, the most efficient way to proceed while still honoring the
> > user-supplied limit on RAM.
>
> Yeah, it can update half the reflinks and pause for a commit, or similar.
> If there's a power failure then there will be a duplicate extent with some
> of the references to one copy and some to the other, but this is probably
> rare enough not to matter.
>
> > > Reflinks can be used to multiply that 12GB arbitrarily--you only get
> > > locality if the reflinks are consecutive in (inode, offset) space,
> > > so if the reflinks are scattered across subvols or files, they won't
> > > share pages.
> >
> > OK.
> >
> > Yes, given a sufficiently pathological case, the defrag will take forever.
> > There is nothing unexpected there. I agree on that point. The defrag always
> > functions within certain prerequisites.
> >
> > > > The trick, in this case, is to select one part of "ultralog" which is
> > > > localized in the metadata, and defragment it. Repeating this step will
> > > > ultimately defragment the entire file.
> > > >
> > > > So, the defrag selects some part of metadata which is entirely a descendant
> > > > of some b-tree node not far from the bottom of b-tree. It selects it such
> > > > that the required update to the metadata is less than, let's say, 64 MB, and
> > > > simultaneously the affected "ultralog" file fragments total less han 512 MB
> > > > (therefore, less than 128 thousand metadata leaf entries, each pointing to a
> > > > 4 KB fragment). Then it finds all the file extents pointed to by that part
> > > > of metadata. They are consecutive (as file fragments), because we have
> > > > selected such part of metadata. Now the defrag can safely copy-move those
> > > > fragments to a new area and update the metadata.
> > > >
> > > > In order to quickly select that small part of metadata, the defrag needs a
> > > > metatdata cache that can hold somewhat more than 128 thousand localized
> > > > metadata leaf entries. That fits into 128 MB RAM definitely.
> > > >
> > > > Of course, there are many other small issues there, but this outlines the
> > > > general procedure.
> > > >
> > > > Problem solved?
> >
> > > Problem missed completely. The forward reference updates were the only
> > > easy part.
> >
> > Oh, I'll reply in another mail, this one is getting too tireing.
> >
> >