DragonFly users List (threaded) for 2008-01
DragonFly BSD
DragonFly users List (threaded) for 2008-01
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Re: Futures - HAMMER comparison testing?

From: Bill Hacker <wbh@xxxxxxxxxxxxx>
Date: Fri, 18 Jan 2008 10:51:44 +0000

Matthew Dillon wrote:
:But - at the end of the day - how much [extra?] on-disk space will be :needed to insure mount 'as-of' is 'good enough' for some realisitic span :(a week?, a month?)? 'Forever' may be too much to ask.

    The amount of disk needed is precisely the same as the amount of
    historical data (different from current data) that must be retained,
    plus record overhead.

    So it comes down to how much space you are willing to eat up to store
    the history, and what kind of granularity you will want for the history.

OK - so it WILL be a 'tunable', then.

FWIW - my yardsticks at the 'heavy' or most wasteful end are punch card & paper/mylar tape on low/no RAM systems, where 'backup' is essentially of 'infinite' granularity, moving through WORM storage to Plan9 Venti, et al.

AFAIK, none of the oldest 'write once' methods are in even 'virtualized' use - save possibly in the FAA or military fields, as few entities have any prectical use for that sort of history.

At the other end, one of our projects involved storing the floor plans of 60,000 buildings on RAID1. A technician manually rebuiding a failed array mirrored empty HDD to full, and over 600 CD's had to be manually reloaded.

In that case, there never had been risk of loss - anyone could buy the latast CD's from the government lands department.

What his error cost us was 'only' time and inconvenience.

HAMMER cannot protect against all forms of human error - BUT - if it inherently rebuilds more intelligently than the least-intelligent of RAID1, it can greatly reduce the opportunity for that sort of 'accident' to occur.

:How close are we to being able to start predicting that storage-space :efficiency relative to ${some_other_fs}?

    Ultimately it will be extremely efficient simply by the fact that
    there will be a balancer going through it and repacking it.

"... constantly, and in the background..." (I presume)

".. and with tunable frequency and priority." (I wish, eventually).

For the moment (and through the alpha release) it will be fairly
inefficient because it is using fixed 16K data records, even for small
files. The on-disk format doesn't care... records can reference variable-length data from around 1MB down to 64 bytes. But supporting
variable-length data requires implementing some overwrite cases that
I don't want to do right now.

Is variable-length still likely to have a payback if the data records were to be fixed at 512B or 1024B or integer multiples thereof?

> This only applies to regular files
of course. Directories store directory entries as records, not as data,
so directories are packed really nicely.

    e.g. if you have one record representing, say, 1MB of data, and you
    write 64 bytes right smack in the middle of that, the write code will
    have to take that one record, mark it as deleted, then create three
    records to replace it (one pointing to the unchanged left portion of
    the original data, one pointing to the 64 bytes of overwritten data,
    and one pointing to the unchanged right portion of the original data).
    The recovery and deletion code will also have to deal with that sort
    of overlayed data situation.  I'm not going to be writing that
    feature for a bit.  There are some quick hacks I can do too, for
    small files, but its not on my list prior to the alpha release.

    Remember that HAMMER is designed for large filesystems which don't fill
    up instantly.  Consequently it will operate under the assumption that
    it can take its time to recover free space.  If one doesn't want to use
    the history feature one can turn it off, of course, or use a very
    granular retention policy.

    My local backup system is currently using a 730GB UFS partition and it
    is able to backup apollo, crater, and leaf with daily cpdups (using
    the hardlink snapshot trick) going back about 3 months.  In fact, I
    can only fill up that 730GB about half way because fsck runs out of
    memory and fails once you get over around 50 million inodes (mostly
    dependant on the number of directories you have)... on UFS that is.
    I found that out the hard way.

. .which reminds us what we will ALL soon face if we do NOT seek newer solutions!

    It takes almost a day for fsck to
    recover the filesystem even half full.  I'll be happy when I can throw
    that old stuff away.

Matthew Dillon <dillon@backplane.com>

. . or just relegate it to what it still does faster/better. IF..

I hope and trust that DragonFly BSD will earn a place as a 'broad spectrum' OS, competitive across the board with alternatives.

But - if not, or even just 'not at first'

- much as OpenBSD and NetBSD have long been seen as good choices for routers and firewalls, DragonFly should be able to carve out a viable niche as the better choice for centralized / clustered / shared-use servers on the basis of:

- superior storage management

- cleaner kernel virtualization

- the very extensive code audit & cleanup that has been ongoing since day one


 , able to  entrant.
But - if not

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