Soft Updates Help

[Garrett Cooper]

Garrett Cooper wrote:
> Bill Moran wrote:
>> Sean Murphy <smurphy at calarts.edu> wrote:
>>> I have read up on soft updates and have some questions.
>>>
>>> The way that I am understanding soft updates purpose is to allow file 
>>> systems to be mounted dirty after an unclean shutdown of the system.
>>
>> That's not the purpose.  The purpose is to improve performance by taking
>> advantage of delayed writes much the way an asynchronous filesystem does,
>> while preventing horrendous data corruption by ordering those writes, 
>> much
>> the way a journalling filesystem does.
>>
>> The fact that you can generate filesystem snapshots is a 
>> side-benefit.  The
>> fact that you can use filesystem snapshots to validate the filesystem 
>> after
>> it's been mounted is a further side-benefit.
>>
>>> If this is a safe way to restore consistency why is it not used on /?
>>
>> Because writes are delayed, it's possible for data to be lost in the 
>> event of
>> a crash -- it acts like a database, either the entire transaction is 
>> committed
>> or it's rolled back, either way, the data is guaranteed not to be 
>> corrupt.
>>
>> Also, on heavily used filesystems, softupdates can lead to the filesystem
>> temporarily having less space available than it really does.  I.e. you 
>> update
>> /kernel, softupdates completely replaces the file with a new one, but the
>> blocks for the old file haven't been reclaimed yet.  For a short 
>> period, you
>> might have 1 kernel file, but there's 2x that being allocated for it.
>>
>> For these two reasons, / is traditionally _not_ mounted with softupdates
>> enabled, since it's critical to system startup.
>>
>>> If a file system is not heavily written to is it better not to use 
>>> soft updates?
>>
>> Weigh the good vs. the bad:
>> *) synchronous mounted filesystem is almost guaranteed to keep your 
>> data safe
>>    at all times, but is abysmally slow.
>> *) softupdates _may_ lose some data if your system crashes before all 
>> writes
>>    are flushed, but will never _corrupt_ it.  Additionally, you get a LOT
>>    better speed.
>> *) Asynchronous is a little faster than softupdates, but it's damn near
>>    guaranteed to be corrupt in the event of a crash.
>>
>>> When file systems are mounted dirty and our being used while the 
>>> backgound fsck is running on the file systems how does it prevent 
>>> files from being lost?
>>
>> It doesn't.  It guarantees that your filesystem will always be 
>> mountable and
>> never corrupt, but it doesn't guarantee against data loss.
>>
>> Here's a simplified example:
>> Let's say you're saving a big file and the power goes out.  When the 
>> power comes
>> back on, there are basically 3 states that file can be in:
>> A) It was fully written to disk -- you got lucky.
>> B) Nothing had been written to disk yet -- "data loss"
>> C) It was partially written to disk -- your filesystem is corrupt, you 
>> either
>>    need to allow a filesystem repair program to fix it (fsck -- or 
>> chkdsk on
>>    Windows, for example) or you'll have weird problems with it until 
>> you do so.
>>
>> Softupdates guarantees against C.  It does this by (essentially) 
>> writing the
>> file "backwards":
>> 1) it writes all the data to data blocks, and once that's done
>> 2) _then_ it creates a directory entry for the file.
>>
>> If the system crashes between #1 and #2, it looks like B happened, but 
>> you never
>> get in scenario C where the filesystem is corrupt and gets more 
>> corrupt as you
>> continue to use it.  Instead, when fsck runs (in the background) it 
>> realizes
>> that there are data blocks in use that don't belong to any file, and 
>> it can
>> free them up for the filesystem to use.
>>
>> That's somewhat simplified, but it gives you the basic idea.
>>
>> HTH
>> Bill
> 
>     Just for future reference, a more brief-although not 
> complete-explanation of softupdates can be found on Wikipedia 
> <http://en.wikipedia.org/wiki/Softupdates>.
>     The article does link some other documents which discuss softupdates 
> in more detail though. From what I skimmed it appears that the documents 
> describe softupdates as a system, but perhaps not all of the features 
> that you are looking for (asynchronous softupdating, for instance).
> -Garrett

More information about softupdates straight from the Handbook (see the 
bottom of the page): 
<http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/configtuning-disk.html>
-Garrett