Lines Matching +full:atomic +full:- +full:threshold +full:- +full:us
1 .. SPDX-License-Identifier: GPL-2.0
27 are atomic and recoverable. For reasons of space and time efficiency, the
33 details logged are made up of the changes to in-core structures rather than
34 on-disk structures. Other objects - typically buffers - have their physical
35 changes logged. Long running atomic modifications have individual changes
60 commit boundaries, whilst "one shot" transactions are for a single atomic
64 place. This means that permanent transactions can be used for one-shot
65 modifications, but one-shot reservations cannot be used for permanent
68 In the code, a one-shot transaction pattern looks somewhat like this::
97 While this might look similar to a one-shot transaction, there is an important
108 this provides a mechanism for complex changes to appear atomic from an external
112 transaction does not form an atomic change in the journal. While each
113 individual modification is atomic, the chain is *not atomic*. If we crash half
123 the on-disk journal.
165 transaction, we have to reserve enough space to record a full leaf-to-root split
177 This "worst case" calculation provides us with the static "unit reservation"
183 For one-shot transactions, a single unit space reservation is all that is
190 transaction rolling mechanism to re-reserve space on every transaction roll. We
194 For example, an inode allocation is typically two transactions - one to
205 means we can roll the transaction multiple times before we have to re-reserve
210 re-reserve physical space in the log. This is somewhat complex, and requires
219 of a cycle number - the number of times the log has been overwritten - and the
233 reservations currently held by active transactions. It is a purely in-memory
251 - and it mostly does track exactly the same location as the reserve grant head -
265 To take a new reservation without sleeping requires us to be able to take a
269 grant head does not track physical space - it only accounts for the amount of
278 xfs_trans_commit() calls, while the physical log space reservation - tracked by
279 the write head - is then reserved separately by a call to xfs_log_reserve()
287 "Re-logging" the locked items on every transaction roll ensures that the items
292 move the tail of the log forwards to free up write grant space. Re-logging the
294 making cannot self-deadlock.
303 Re-logging Explained
309 method called "re-logging". Conceptually, this is quite simple - all it requires
333 of each subsequent transaction, and it's the technique that allows us to
334 implement long-running, multiple-commit permanent transactions.
347 the log - repeated operations to the same objects write the same changes to
357 in memory - batching them, if you like - to minimise the impact of the log IO on
362 buffers available and the size of each is 32kB - the size can be increased up
365 Effectively, this gives us the maximum bound of outstanding metadata changes
366 that can be made to the filesystem at any point in time - if all the log
383 but only one of those copies needs to be there - the last one "D", as it
402 actually relatively easy to do - all the changes to logged items are already
438 4. No on-disk format change (metadata or log format).
446 ---------------
453 require us to lock every object, format them, and then unlock them again.
463 The solution is relatively simple - it just took a long time to recognise it.
474 that does not require us to lock the item to access. This formatting and
486 Object +---------------------------------------------+
487 Vector 1 +----+
488 Vector 2 +----+
489 Vector 3 +----------+
493 Log Buffer +-V1-+-V2-+----V3----+
497 Object +---------------------------------------------+
498 Vector 1 +----+
499 Vector 2 +----+
500 Vector 3 +----------+
504 Memory Buffer +-V1-+-V2-+----V3----+
505 Vector 1 +----+
506 Vector 2 +----+
507 Vector 3 +----------+
518 buffer writing (i.e. double encapsulation). This would be an on-disk format
525 self-describing object that can be passed to the log buffer write code to be
527 Hence we avoid needing a new on-disk format to handle items that have been
532 ----------------
543 and as such are stored in the Active Item List (AIL) which is a LSN-ordered
561 its place in the list and re-inserted at the tail. This is entirely arbitrary
562 and done to make it easy for debugging - the last items in the list are the
569 ----------------------------
574 need to be written as an atomic transaction. The need for all the objects to be
575 written as an atomic transaction comes from the requirements of relogging and
576 log replay - all the changes in all the objects in a given transaction must
594 to any other transaction - it contains a transaction header, a series of
596 perspective, the checkpoint transaction is also no different - just a lot
604 transaction. The current log write code enables us to do this easily with the
606 the transaction commit record, but tracking this requires us to have a
607 per-checkpoint context that travels through the log write process through to
617 This allows us to unlock the CIL immediately after transfer of all the
638 Log Item <-> log vector 1 -> memory buffer
639 | -> vector array
641 Log Item <-> log vector 2 -> memory buffer
642 | -> vector array
647 Log Item <-> log vector N-1 -> memory buffer
648 | -> vector array
650 Log Item <-> log vector N -> memory buffer
651 -> vector array
659 log vector 1 -> memory buffer
660 | -> vector array
661 | -> Log Item
663 log vector 2 -> memory buffer
664 | -> vector array
665 | -> Log Item
670 log vector N-1 -> memory buffer
671 | -> vector array
672 | -> Log Item
674 log vector N -> memory buffer
675 -> vector array
676 -> Log Item
703 --------------------------------------
710 re-using a freed metadata extent for a data extent), a special, optimised log
720 As discussed in the checkpoint section, delayed logging uses per-checkpoint
725 atomic counter - we can just take the current context sequence number and add
754 else for such serialisation - it only matters when we do a log force.
767 ------------------------------------------------
785 inode changes. If you modify lots of inode cores (e.g. ``chmod -R g+w *``), then
792 buffer format structure for each buffer - roughly 800 vectors or 1.51MB total
810 reservation of around 150KB, which is a non-trivial amount of space.
812 A static reservation needs to manipulate the log grant counters - we can take a
843 the maximum threshold, we need to push the CIL to the log. This is effectively
859 ---------------------------------
875 That is, we now have a many-to-one relationship between transaction commit and
883 pin the object the first time it is inserted into the CIL - if it is already in
900 ---------------------------------------
910 points in the design - the three important ones are:
917 that we have a many-to-one interaction here. That is, the only restriction on
924 relatively long period of time - the pinning of log items needs to be done
932 really needs to be a sleeping lock - if the CIL flush takes the lock, we do not
941 compared to transaction commit for asynchronous transaction workloads - only
942 time will tell if using a read-write semaphore for exclusion will limit
979 -----------------
1019 Essentially, steps 1-6 operate independently from step 7, which is also
1020 independent of steps 8-9. An item can be locked in steps 1-6 or steps 8-9
1021 at the same time step 7 is occurring, but only steps 1-6 or 8-9 can occur
1023 and steps 1-6 are re-entered, then the item is relogged. Only when steps 8-9
1075 logging methods are in the middle of the life cycle - they still have the same
1081 As a result of this zero-impact "insertion" of delayed logging infrastructure