mirror of
https://git.deuxfleurs.fr/Deuxfleurs/garage.git
synced 2024-11-14 04:01:25 +00:00
38 lines
3.3 KiB
Markdown
38 lines
3.3 KiB
Markdown
## Context
|
|
|
|
Data storage is critical: it can lead to data loss if done badly and/or on hardware failure.
|
|
Filesystems + RAID can help on a single machine but a machine failure can put the whole storage offline.
|
|
Moreover, it put a hard limit on scalability. Often this limit can be pushed back far away by buying expensive machines.
|
|
But here we consider non specialized off the shelf machines that can be as low powered and subject to failures as a raspberry pi.
|
|
|
|
Distributed storage may help to solve both availability and scalability problems on these machines.
|
|
Many solutions were proposed, they can be categorized as block storage, file storage and object storage depending on the abstraction they provide.
|
|
|
|
## Related work
|
|
|
|
Block storage is the most low level one, it's like exposing your raw hard drive over the network.
|
|
It requires very low latencies and stable network, that are often dedicated.
|
|
However it provides disk devices that can be manipulated by the operating system with the less constraints: it can be partitioned with any filesystem, meaning that it supports even the most exotic features.
|
|
We can cite [iSCSI](https://en.wikipedia.org/wiki/ISCSI) or [Fibre Channel](https://en.wikipedia.org/wiki/Fibre_Channel).
|
|
Openstack Cinder proxy previous solution to provide an uniform API.
|
|
|
|
File storage provides a higher abstraction, they are one filesystem among others, which means they don't necessarily have all the exotic features of every filesystem.
|
|
Often, they relax some POSIX constraints while many applications will still be compatible without any modification.
|
|
As an example, we are able to run MariaDB (very slowly) over GlusterFS...
|
|
We can also mention CephFS (read [RADOS](https://ceph.com/wp-content/uploads/2016/08/weil-rados-pdsw07.pdf) whitepaper), Lustre, LizardFS, MooseFS, etc.
|
|
OpenStack Manila proxy previous solutions to provide an uniform API.
|
|
|
|
Finally object storages provide the highest level abstraction.
|
|
They are the testimony that the POSIX filesystem API is not adapted to distributed filesystems.
|
|
Especially, the strong concistency has been dropped in favor of eventual consistency which is way more convenient and powerful in presence of high latencies and unreliability.
|
|
We often read about S3 that pioneered the concept that it's a filesystem for the WAN.
|
|
Applications must be adapted to work for the desired object storage service.
|
|
Today, the S3 HTTP REST API acts as a standard in the industry.
|
|
However, Amazon S3 source code is not open but alternatives were proposed.
|
|
We identified Minio, Pithos, Swift and Ceph.
|
|
Minio/Ceph enforces a total order, so properties similar to a (relaxed) filesystem.
|
|
Swift and Pithos are probably the most similar to AWS S3 with their consistent hashing ring.
|
|
However Pithos is not maintained anymore. More precisely the company that published Pithos version 1 has developped a second version 2 but has not open sourced it.
|
|
Some tests conducted by the [ACIDES project](https://acides.org/) have shown that Openstack Swift consumes way more resources (CPU+RAM) that we can afford. Furthermore, people developing Swift have not designed their software for geo-distribution.
|
|
|
|
There were many attempts in research too. I am only thinking to [LBFS](https://pdos.csail.mit.edu/papers/lbfs:sosp01/lbfs.pdf) that was used as a basis for Seafile. But none of them have been effectively implemented yet.
|