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 <ce3g8...@umail.furryterror.org>: > > > > > On Fri, Sep 13, 2019 at 01:05:52AM -0400, General Zed wrote: > > > > > > > > Quoting Zygo Blaxell <ce3g8...@umail.furryterror.org>: > > > > > > > > > On Thu, Sep 12, 2019 at 08:26:04PM -0400, General Zed wrote: > > > > > > > > > > > > Quoting Zygo Blaxell <ce3g8...@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. > > > >
