Guys, fisrt of all, archiving is not for PITR at all, this is optimization. If we disable archiving, every rollover we need to create new file. If we enable archiving, we reserve 10 (by default) segments filled with zeroes. We use mmap by default, so if we use no-archiver approach: 1. We firstly create new empty file 2. Call on it sun.nio.ch.FileChannelImpl#map, thats under the hood a. If file is shorter, than wal segment size, it calls sun.nio.ch.FileDispatcherImpl#truncate0, this is under the hood just a system call truncate [1] b. Than it calls system call mmap on this file sun.nio.ch.FileChannelImpl#map0, under the hood see [2] These manipulation are not free and cheap. So rollover will be much much slower. If archiving is enabled, 10 segments are already preallocated at the moment of node's start.
When archiving is enabled, archiver just copy previous preallocated segment and move it to archive directory. This archived segment is crucial for recovery. When new checkpoints finished, all eligible for trunocating segments are just removed. If archiving is disabled, we also write WAL segments in wal directory and disabling archiving don't prevent you from storing segments, if they are required for recovery. >>Before increasing the size of WAL archive (transferring to archive /rollOver, compression, decompression), we can make sure that there will be enough space in the archive and if there is no such, then we will try to >>clean it. We cannot delete those segments that are required for recovery (between the last two checkpoints) and reserved for example for historical rebalancing. First of all, compression/decompression is offtopic here. Secondly, wal segments are required only with idx higher than LAST checkpoint marker. Thirdly, archiving and rolling over can be during checkpoint and we can broke everything accidentially. Fourthly, I see no benefits to overcomplicated already complicated logic. This is basically problem of misunderstanding and tuning. There are a lot of similar topics for almost every DB. [3] [1] -- https://man7.org/linux/man-pages/man2/ftruncate.2.html [2] -- https://man7.org/linux/man-pages/man2/mmap.2.html [3] -- https://www.google.com/search?q=pg_wal%2Fxlogtemp+no+space+left+on+device&oq=pg+wal+no пт, 6 нояб. 2020 г. в 10:42, ткаленко кирилл <tkalkir...@yandex.ru>: > Hi, Ivan! > > I have only described ideas. But here are a few more details. > > We can take care not to go beyond > DataStorageConfiguration#maxWalArchiveSize. > > Before increasing the size of WAL archive (transferring to archive > /rollOver, compression, decompression), we can make sure that there will be > enough space in the archive and if there is no such, then we will try to > clean it. We cannot delete those segments that are required for recovery > (between the last two checkpoints) and reserved for example for historical > rebalancing. > > We can receive a notification about the change of checkpoints and the > reservation / release of segments, thus we can know how many segments we > can delete right now. > > 06.11.2020, 09:53, "Ivan Daschinsky" <ivanda...@gmail.com>: > >>> For example, when trying to move a segment to the archive. > > > > We cannot do this, we will lost data. We can truncate archived segment if > > and only if it is not required for recovery. If last checkpoint marker > > points to segment > > with lower index, we cannot delete any segment with higher index. So the > > only moment where we can remove truncate segments is a finish of > checkpoint. > > > > пт, 6 нояб. 2020 г. в 09:46, ткаленко кирилл <tkalkir...@yandex.ru>: > > > >> Hello, everybody! > >> > >> As far as I know, WAL archive is used for PITP(GridGain feature) and > >> historical rebalancing. > >> > >> Facundo seems to have a problem with running out of directory > >> (/opt/work/walarchive) space. > >> Currently, WAL archive is cleared at the end of checkpoint. Potentially > >> long transaction may prevent checkpoint starting, thereby not cleaning > WAL > >> archive, which will lead to such an error. > >> At the moment, I see such a WA to increase size of directory > >> (/opt/work/walarchive) in k8s and avoid long transactions or something > like > >> that that modifies data and runs for a long time. > >> > >> And it is best to fix the logic of working with WAL archive. I think we > >> should remove WAL archive cleanup from the end of the checkpoint and > do it > >> on demand. For example, when trying to move a segment to the archive. > >> > >> 06.11.2020, 01:58, "Denis Magda" <dma...@apache.org>: > >> > Folks, > >> > > >> > In my understanding, you need the archives only for features such as > >> PITR. > >> > Considering, that the PITR functionality is not provided in Ignite > why do > >> > we have the archives enabled by default? > >> > > >> > How about having this feature disabled by default to prevent the > >> following > >> > issues experienced by our users: > >> > > >> > http://apache-ignite-users.70518.x6.nabble.com/WAL-and-WAL-Archive-volume-size-recommendation-td34458.html > >> > > >> > - > >> > Denis > > > > -- > > Sincerely yours, Ivan Daschinskiy > -- Sincerely yours, Ivan Daschinskiy
