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

[Fabian Ebner]

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


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,
> +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"?

> +
> +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."?

> +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
>   ---------------
>