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Difference between revisions of "Btrfs"
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* Files are appended at the end of a "data page" | * Files are appended at the end of a "data page" | ||
* Metadata is appended at a "metadata page" | * Metadata is appended at a "metadata page" | ||
* Inside a page nothing is ever overwritten | * Inside a page '''nothing is ever overwritten''' | ||
* When a page is full the file system will use the next free page | * When a page is full the file system will use the next free page | ||
* Deleting a file does not write/clean its data, but writes metadata, that marks this file as deleted | * Deleting a file does not write/clean its data, but writes metadata, that marks this file as deleted | ||
* Overwriting a file does first append the new file to the "data page", then | * Overwriting a file does first append the new file to the actual "data page", then appends the metadata for this file to the "metadata page". | ||
* Changing small parts of a file will write only the new parts, then link the rest to the old file | * Changing small parts of a file will write only the new parts, then ''link'' the rest to the old file | ||
* there are checksums for data and metadata | * there are checksums for data and metadata | ||
==== Downsides ==== | ==== Downsides ==== | ||
* Management of space is complex | * Management of space is complex | ||
* There are 2 sorts of pages | * There are 2 sorts of pages (data / metadata) | ||
* There has to be a clean-up-process who makes the space of deleted files reusable, so that the disk does not run out of free pages | * There has to be a clean-up-process who makes the space of deleted files reusable, so that the disk does not run out of free pages | ||
* It must be avoided to write data unnecessarily, because then the clean-up would also be very expensive | * It must be avoided to write data unnecessarily, because then the clean-up would also be very expensive | ||
==== | ==== Advantages ==== | ||
* It is possible to detect nearly any corruption because of the checksums | * It is possible to '''detect''' nearly any '''corruption''' because of the checksums | ||
* When the power is lost, or the disk is disconnected, all old data is save. WHY? | |||
** Every bit of "old" data from before the power loss or the disconnection is present because it is NOT overwritten | ** Every bit of "old" data from before the power loss or the disconnection is present because it is '''NOT overwritten''' | ||
** Only the newly written data may be partly damaged | ** Only the newly written data may be partly damaged | ||
** The metadata may also be partly damaged | ** The metadata may also be partly damaged | ||
** When mounting the volume it is possible by | ** When mounting the volume it is possible by analyzing checksums and metadata to find the point in the filesystem where all was good | ||
** Btrfs will automatically roll back to this point, then it can mount the file system | ** Btrfs will '''automatically roll back''' to this point, then it can mount the file system writable | ||
* CoW is a sound foundation to build upon | * CoW is a sound foundation to build upon | ||