[pbs-devel] [PATCH proxmox-backup v2] docs: explain some technical details about datastores/chunks

[Dylan Whyte]

On 12/10/20 1:56 PM, Fabian Ebner wrote:
> I think it needs to be "fixed-size" instead of "fixed-sized". Since 
> "dynamically" is an adverb, "dynamically-sized" is fine OTOH.
>
> @Dylan: please correct me if I'm wrong
"fixed-sized chunks" is correct, as "-sized" is an adjective describing 
the chunks. But now that you mention this, it should probably be 
"dynamically sized" (no hyphen), because in that case, like you said, 
it's an adverb.
>
>
> More nits/suggestions inline.
>
> Am 10.12.20 um 11:19 schrieb Dominik Csapak:
>> adds explanations for:
>> * what datastores are
>> * their relation with snapshots/chunks
>> * basic information about chunk directory structures
>> * fixed-/dynamically-sized chunks
>> * special handling of encrypted chunks
>> * hash collision probability
>> * limitation of file-based backups
>>
>> Signed-off-by: Dominik Csapak <d.csapak at proxmox.com>
>> ---
>> changes from v1:
>> * incorporated Oguz suggestions
>> * added more details to
>>   - deduplication info
>>   - fixed-sized chunks and snaphots
>>   - dynamically-sized chunks and pxar format
>>
>>   docs/index.rst              |   1 +
>>   docs/technical-overview.rst | 166 ++++++++++++++++++++++++++++++++++++
>>   docs/terminology.rst        |   3 +
>>   3 files changed, 170 insertions(+)
>>   create mode 100644 docs/technical-overview.rst
>>
>> diff --git a/docs/index.rst b/docs/index.rst
>> index fffcb4fd..f3e6bf0c 100644
>> --- a/docs/index.rst
>> +++ b/docs/index.rst
>> @@ -33,6 +33,7 @@ in the section entitled "GNU Free Documentation 
>> License".
>>      pve-integration.rst
>>      pxar-tool.rst
>>      sysadmin.rst
>> +   technical-overview.rst
>>      faq.rst
>>     .. raw:: latex
>> diff --git a/docs/technical-overview.rst b/docs/technical-overview.rst
>> new file mode 100644
>> index 00000000..fa6d8e7b
>> --- /dev/null
>> +++ b/docs/technical-overview.rst
>> @@ -0,0 +1,166 @@
>> +Technical Overview
>> +==================
>> +
>> +.. _technical_overview:
>> +
>> +Datastores
>> +----------
>> +
>> +A Datastore is the logical place where :ref:`Backup Snapshots 
>> <backup_snapshot>`
>> +and their chunks are stored. Snapshots consist of a manifest, blobs,
>> +dynamic- and fixed-indexes (see :ref:`terminology`), and are stored 
>> in the
>> +following directory structure:
>> +
>> + <datastore-root>/<type>/<id>/<time>/
>> +
>> +The deduplication of datastores is based on reusing chunks, which are
>> +referenced by the indexes in a backup snapshot. This means that 
>> multiple
>> +indexes can reference the same chunks, reducing the amount of space
>> +needed to contain the data (even across backup snapshots).
>> +
>> +Chunks
>> +------
>> +
>> +A chunk is some (possibly encrypted) data with a CRC-32 checksum at
>> +the end and a type marker at the beginning. It is identified by a
>> +SHA-256 checksum of its content.
>
> a SHA-256 checksum -> the SHA-256 checksum
>
>> +
>> +To generate such chunks, backup data is split either into fixed-size or
>> +dynamically-sized chunks. The same content will be hashed to the same
>> +checksum.
>> +
>> +The chunks of a datastore are found in
>> +
>> + <datastore-root>/.chunks/
>> +
>> +This chunk directory is further subdivided by the first four byte of 
>> the
>> +chunks checksum, so the chunk with the checksum
>> +
>> + a342e8151cbf439ce65f3df696b54c67a114982cc0aa751f2852c2f7acc19a8b
>> +
>> +lives in
>> +
>> + <datastore-root>/.chunks/a342/
>> +
>> +This is done to reduce the number of files per directory, as having
>> +many files per directory can be bad for file system performance.
>> +
>> +These chunk directories('0000'-'ffff') will be preallocated when a 
>> datastore is
>
> missing space before the parenthesis?
>
>> +created.
>> +
>> +Fixed-sized Chunks
>> +^^^^^^^^^^^^^^^^^^
>> +
>> +For block based backups (like VMs), fixed-sized chunks are used. The 
>> content
>> +(disk image), is split into chunks of the same length (typically 4 
>> MiB).
>> +
>> +This works very well for VM images, since the file system on the guest
>> +most often tries to allocate files in contiguous pieces, so new 
>> files get
>> +new blocks, and changing existing files changes only their own blocks.
>> +
>> +As an optimization, VMs in `Proxmox VE`_ can make use of 'dirty 
>> bitmaps',
>> +which can track the changed blocks of an image. Since these bitmap
>> +are also a representation of the image split into chunks, we have
>> +a direct relation between dirty blocks of the image and chunks we have
>> +to upload, so only modified chunks of the disk have to be uploaded 
>> for a backup.
>> +
>> +Since we always split the image into chunks of the same size, 
>> non-changed
>
> non-changed -> unchanged
>
>> +blocks will result in identical checksums for those chunks, which do 
>> not
>
> which do not -> so such chunks do not
>
>> +need to be backed up again. This way storage snapshots are not 
>> needed and used
>> +to find the changed blocks.
>
> Get rid of the "and used"?
>
>> +
>> +For consistency, `Proxmox VE`_ uses a QEMU internal snapshot 
>> mechanism, that
>> +does not rely on storage snapshots either.
>> +
>> +Dynamically-sized Chunks
>> +^^^^^^^^^^^^^^^^^^^^^^^^
>> +
>> +If one does not want to backup block-based systems but file-based 
>> systems,
Change "but" to "but rather".
>> +using fixed-sized chunks is not a good idea, since every time a file
>> +would change in size, the remaining data gets shifted around and this
>> +would result in many chunks changing, reducing the effect of 
>> deduplication.
>
> Remove "the effect of"?
Or change to "amount of deduplication."
>> +
>> +To improve this, `Proxmox Backup`_ Server uses dynamically-sized chunks
>> +instead. Instead of splitting an image into fixed sizes, it first 
>> generates
>> +a consistent file archive (:ref:`pxar <pxar-format>`) and uses a 
>> rolling hash
>> +over this on-the-fly generated archive to calculate chunk boundaries.
>> +
>> +We use a variant of Buzhash which is a cyclic polynomial algorithm.
>> +It works by continuously calculating a checksum while iterating over 
>> the
>> +data, and on certain conditions it triggers a hash boundary.
>> +
>> +Assuming that most files of the system that is to be backed up have 
>> not changed,
>> +eventually the algorithm triggers the boundary on the same data as a 
>> previous
>> +backup, resulting in chunks that can be reused.
>> +
>> +Encrypted Chunks
>> +^^^^^^^^^^^^^^^^
>> +
>> +A special case are encrypted chunks. Both fixed- and dynamically-sized
>
> The "A special case are" sounds strange IMHO. Use "Encrypted chunks 
> are special."?
Would prefer "Encrypted chunks are a special case", if changing.
>
>> +chunks can be encrypted, and their handling is slightly different
>
> "their handling is slightly different" -> "they are handled in a 
> slightly different manner"
>
>> +from normal chunks.
>> +
>> +The hash of encrypted chunks are calculated not with the actual 
>> (encrypted)
>
> hash -> hashes
>
>> +chunk content, but with the plaintext content concatenated with
>> +the encryption key. This way, two chunks of the same data encrypted 
>> with
>> +different keys generate two different checksums and no collisions 
>> occur for
>> +multiple encryption keys.
>> +
>> +This is done to speed up the client part of the backup, since it 
>> only needs
>> +to encrypt chunks that are actually getting uploaded and chunks that 
>> exist
>
> "and chunks" -> ". Chunks"
> exist already -> already exist?
>
>> +already in the previous backup, do not need to be encrypted and 
>> uploaded.
>> +
>> +Caveats and Limitations
>> +-----------------------
>> +
>> +Notes on hash collisions
>> +^^^^^^^^^^^^^^^^^^^^^^^^
>> +
>> +Every hash has a chance to produce collisions, meaning two (or more) 
>> inputs
>
> hash -> hashing algorithm
>
>> +generating the same checksum. For SHA-256, this chance is negligible.
>
> generating -> generate?
>
>> +To calculate such a collision, one can use the ideas of the 
>> 'birthday problem'
>> +from probability theory. For big numbers, this is actually 
>> infeasible to
>> +calculate with regular computers, but there is a good approximation:
>> +
>> +.. math::
>> +
>> + p(n, d) = 1 - e^{-n^2/(2d)}
>> +
>> +Where `n` is the number of tries, and `d` is the number of 
>> possibilities.
>> +So for example, if we assume a large datastore of 1 PiB, and an 
>> average chunk
>> +size of 4 MiB, we have :math:`n = 268435456` tries, and :math:`d = 
>> 2^{256}`
>> +possibilities.  Using the above formula we get that the probability 
>> of a
>> +collision in that scenario is:
>> +
>> +.. math::
>> +
>> + 3.1115 * 10^{-61}
>> +
>> +For context, in a lottery game of 6 of 45, the chance to correctly 
>> guess all
>> +6 numbers is only :math:`1.2277 * 10^{-7}`.
>> +
>> +So it is extremely unlikely that such a collision would occur by 
>> accident
>> +in a normal datastore.
>> +
>> +Additionally, SHA-256 is prone to length extension attacks, but since
>> +there is an upper limit for how big the chunk are, this is not a
>> +problem, since a potential attacker cannot arbitrarily add content to
>> +the data beyond that limit.
>> +
>> +File-based Backup
>> +^^^^^^^^^^^^^^^^^
>> +
>> +Since dynamically-sized chunks (for file-based backups) are created 
>> on a custom
>> +archive format (pxar) and not over the files directly, there is no 
>> relation
>> +between files and the chunks. This means we have to read all files 
>> again
>> +for every backup, otherwise it would not be possible to generate a 
>> consistent
>> +pxar archive where the original chunks can be reused.
>> +
>> +Verification of encrypted chunks
>> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>> +
>> +For encrypted chunks, only the checksum of the original (plaintext) 
>> data
>> +is available, making it impossible for the server (without the 
>> encryption key),
>> +to verify its content against it. Instead only the CRC-32 checksum 
>> gets checked.
>> +
>> +
>> diff --git a/docs/terminology.rst b/docs/terminology.rst
>> index 89ec7646..3eff9780 100644
>> --- a/docs/terminology.rst
>> +++ b/docs/terminology.rst
>> @@ -1,3 +1,5 @@
>> +.. _terminology:
>> +
>>   Terminology
>>   ===========
>>   @@ -99,6 +101,7 @@ Backup Group
>>   The tuple ``<type>/<ID>`` is called a backup group. Such a group
>>   may contain one or more backup snapshots.
>>   +.. _backup_snapshot:
>>     Backup Snapshot
>>   ---------------
>>
>
>
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