Re: [ANNOUNCE] Reiser5: Data Tiering. Burst Buffers. Speedup synchronous modifications

[jose . r . r]

On Mon, May 25, 2020 at 6:08 PM Edward Shishkin <edward.shishkin@xxxxxxxxx> wrote:
>
>                   Reiser5: Data Tiering. Burst Buffers
>                    Speedup synchronous modifications
>
>
>               Dumping peaks of IO load to a proxy device
>
>
> Now you can add a small high-performance block device to your large
> logical volume composed of relatively slow commodity disks and get
> an impression that the whole your volume has throughput which is as
> high, as the one of that "proxy" device!
>
> This is based on a simple observation that in real life IO load is
> going by peaks, and the idea is to dump those peaks to a high-
> performance "proxy" device. Usually you have enough time between peaks
> to flush the proxy device, that is, to migrate the "hot data" from the
> proxy device to slow media in background mode, so that your proxy
> device is always ready to accept a new portion of "peaks".
>
> Such technique, which is also known as "Burst Buffers", initially
> appeared in the area of HPC. Despite this fact, it is also important
> for usual applications. In particular, it allows to speedup the ones,
> which perform so-called "atomic updates".
>
>
>                 Speedup "atomic updates" in user-space
>
>
> There is a whole class of applications with high requirements to data
> integrity. Such applications (typically data bases) want to be sure
> that any data modifications either complete, or they don't. And they
> don't appear as partially occurred. Some applications has weaker
> requirements: with some restrictions they accept also partially
> occurred modifications.
>
> Atomic updates in user space are performed via a sequence of 3 steps.
> Suppose you need to modify data of some file "foo" in an atomic way.
> For this you need to:
>
> 1. write a new temporary file "foo.tmp" with modified data
> 2. issue fsync(2) against "foo.tmp"
> 3. rename "foo.tmp" to "foo".
>
> At step 1 the file system populates page cache with new data
> At step 2 the file system allocates disk addresses for all logical
> blocks of the file foo.tmp and writes that file to disk. At step 3 all
> blocks containing old data get released.
>
> Note that steps 2 and 3 become a reason of essential performance drop
> on slow media. The situation gets improved, when all dirty data are
> written to a dedicated high-performance proxy-disk, which exactly
> happens in a file system with Burst Buffers support.
>
>
>            Speedup all synchronous modifications (TODO)
>                Burst Buffers and transaction manager
>
>
> Not only dirty data pages, but also dirty meta-data pages can be
> dumped to the proxy-device, so that step (3) above also won't
> contribute to the performance drop.
>
> Moreover, not only new logical data blocks can be dumped to the proxy
> disk. All dirty data pages, including ones, which already have
> location on the main (slow) storage can also be relocated to the proxy
> disk, thus, speeding up synchronous modification of files in _all_
> cases (not only in atomic updates via write-fsync-rename sequence
> described above).
>
> Indeed, let's remind that any modified page is always written to disk
> in a context of committing some transaction. Depending on the commit
> strategy (there are 2 ones "relocate" and "overwrite"), for each such
> modified dirty page there are only 2 possibility:
>
> a) to be written right away to a new location,
> b) to be written first to a temporary location (journal), then to be
>     written back to permanent location.
>
> With Burst buffers support in the case (a) the file system writes
> dirty page right away to the proxy device. Then user should take care
> to migrate it back to the permanent storage (see section "Flushing
> proxy devise" below). In the case (b) the modified copy will be
> written to the proxy device (wandering logs), then at checkpoint time
> (playing a transaction) reiser4 transaction manager will write it to
> the permanent location (on commodity disks). In this case user doesn't
> need to worry on flushing proxy device, however, the procedure of
> commit takes more time, as user should also wait for "checkpoint
> completion".
>
> So from the standpoint of performance "write-anywhere" transaction
> model (reiser4 mount option "txmod=wa") is more preferable then
> journalling model (txmod=journal), or even hybrid model (txmod=hybrid)
>
>
>              Predictable and non-predictable migration
>                        Meta-data migration
>
>
> As we already mentioned, not only dirty data pages, but also dirty
> meta-data pages can be dumped to the proxy-device. Note, however, that
> not predictable meta-data migration is not possible because of
> chicken-eggish problem. Indeed, non-predictable migration means that
> nobody knows, on what device of your logical volume a stripe of data
> will be relocated in the future. Such migration requires to record
> location of data stripes. Now note, that such records is always a part
> of meta-data. Hence, you are now able to migrate meta-data in
> non-predictable way.
>
> However, it is perfectly possible to distribute/migrate meta-data in a
> predictable way (it will be supported in so-called "symmetric" logical
> volumes - currently not implemented). Classic example of predictable
> migration is RAID arrays (once you add, or remove a device to/from the
> array, all data blocks migrate in predictable way during rebalancing).
> If relocation is predictable, then it is not need to record locations
> of data stripes - it can always be calculated.
>
> Thus, non-predictable migration is applicable to data only.
>
>
>                    Definition of data tiering.
>               Using proxy device to store hot data (TODO)
>
>
> Now we can precisely define tiering as (meta-)data relocation in
> accordance with some strategy (automatic, or user-defined), so that
> every relocated unit always gets location on another device-component
> of the logical volume.
>
> During such relocation block number B1 on device D1 gets released,
> first address component is changed to D2, second component is changed
> to 0 (which indicates not allocated block number), then the file
> system allocates block number B2 on device D2:
>
>                  (D1, B1) -> (D2, 0) -> (D2, B2)
>
> Note that tiering is not defined for simple volumes (i.e. volumes,
> consisting only of one device). Blocks relocation within one device
> is always in a competence of a file system (to be precisely, of block
> allocator.
>
> Burst buffers is just one of strategies, in accordance with which all
> new logical blocks (optionally, all dirty pages) always get location
> on a dedicated proxy device. As we have figured out, Burst Buffers is
> useful for HPC applications, as well as for usual applications
> executing fsync(2) frequently.
>
> There are other data tiering strategies, which can be useful for other
> class of applications. All of them can be easily implemented in
> Reiser5.
>
> For example, you can use proxy device to store hot data only. With
> such strategy new logical blocks (which are always "cold") will always
> go to the main storage (in contrast with Burst Buffers, where new
> logical blocks first get written to the proxy disk). Once in a while
> you need to scan your volume in order to push colder data out, and
> pull hotter data in the proxy disk. Reiser5 contains a common
> interface for this. It is possible to maintain per-file, or even per-
> blocks-extent "temperature" of data (e.g. as a generation counter),
> but we still don't have more or less satisfactory algorithms to
> determine "critical temperature" for pushing data in/out proxy disk.
>
>
>          Getting started with proxy disk over logical volume
>
>
> Just follow the administration guide:
> https://reiser4.wiki.kernel.org/index.php/Proxy_Device_Administration
>
Re:
> WARNING: THE STUFF IS NOT STABLE! Don't store important data on
> Reiser5 logical volumes till beta-stability announcement.
Will you be releasing reiser4 Software Format Release Number 4.0.2 for Linux kernel 5.6 patch?

>From personal experience SFRN 4.0.2 is stable and all my data, local and cloud virtual machines instances, as well as computing for the last six(6)+ years is in that format/environment. Although I have not tried an Debian based installation with this second iteration of SFRN 5 I have no use for the kernel/reiser4progs until they play well with Debian installer, python, etc..

Best Professional Regards.


-- 
Jose R R
http://metztli.it
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