My opinion, as from others here, is not to change the fractional reserve and leave it at 100%, then to size volume hosting luns at double of their size. And, last, do not use snapshot autodelete: I've seen tremendous thing happening with this option enabled! :-)
The best way to understand the fractional reserve impact changes see the TR of best practice on SnapManager for Exchange 4 and SnapManager for SQL 2000/2005
-----Messaggio originale----- Da: Borzenkov, Andrey [mailto:Andrey.Borzenkov@fujitsu-siemens.com] Inviato: martedì 4 dicembre 2007 17.04 A: Glenn Walker; Parisi, Justin; Holland, William L; Raj Patel Cc: Milazzo Giacomo; toasters@mathworks.com Oggetto: RE: Can't expand SnapDrive for Windows drive
One more possibility with 7.2 is to set fractional reserve to 0%, snap reserve to some reasonable value, let's say 20%, and configure snapshot autodelete feature for volume in question on snap reserve overflow. This will probably combine the best of two worlds :)
С уважением / With best regards / Mit freundlichen Grüβen
--- Andrey Borzenkov Senior system engineer
-----Original Message----- From: Glenn Walker [mailto:ggwalker@mindspring.com] Sent: Tuesday, December 04, 2007 6:56 PM To: Parisi, Justin; Borzenkov, Andrey; Holland, William L; Raj Patel Cc: Milazzo Giacomo; toasters@mathworks.com Subject: RE: Can't expand SnapDrive for Windows drive
What Andrey described is also applicable to Space Reservations\Fractional Reserve. Snap Reserve and Space Reservations essentially serve the same purpose, albeit with different methods:
Snap Reserve reserves a portion of the entire filesystem so that snapshots do not consume active filesystem space (unless they get too large). These are 'per volume'.
Space Reservations reserves a portion of the filesystem _per_file_ so that snapshots do not consume active filesystem space that is needed for an 'overwrite' (WAFL doesn't technically overwrite, ever) of a consumed block in a file\LUN.
Justin, what you described is most certainly why NetApp recommends 100% space reserve - if you have every block in a LUN filled (even if only zeroed out by the attached host, it's still consumed) and take a snapshot, an attempt to overwrite every block within that LUN would be met with an ENOSPACE error if 100% is not available, effectively causing data corruption due to non-committed write.
If you have a proactive environment, you can set the fractional reserve to less than 100% and keep close watch to add space when needed, thereby reducing overall storage consumption/cost initially. If you have a reactive environment, just keep it at 100% and sleep well at night.
Glenn
-----Original Message----- From: owner-toasters@mathworks.com [mailto:owner-toasters@mathworks.com] On Behalf Of Parisi, Justin Sent: Tuesday, December 04, 2007 9:03 AM To: Borzenkov, Andrey; Holland, William L; Raj Patel Cc: Milazzo Giacomo; toasters@mathworks.com Subject: RE: Can't expand SnapDrive for Windows drive
I think you're thinking of snap reserve, which is an entirely different beast.
Fractional reserve is set to 100% by default. The reason for this is to set aside enough space if you were to overwrite an entire lun. If that happened, your snapshot would be as large as the lun itself. Now, every environment is different, so you can adjust the fractional reserve accordingly, but NetApp best practice is to leave it at 100%.
-----Original Message----- From: Borzenkov, Andrey [mailto:Andrey.Borzenkov@fujitsu-siemens.com] Sent: Tuesday, December 04, 2007 1:50 AM To: Holland, William L; Raj Patel Cc: Milazzo Giacomo; toasters@mathworks.com Subject: RE: Can't expand SnapDrive for Windows drive
You should understand implications of it. Fractional reserve allows you to cheat. But if you set it to 10% and in reality need 20%, writing to LUN will fail unless snapshots are deleted (can be done automatically in new versions).
С уважением / With best regards / Mit freundlichen Grüβen
--- Andrey Borzenkov Senior system engineer -----Original Message----- From: owner-toasters@mathworks.com [mailto:owner-toasters@mathworks.com] On Behalf Of Holland, William L Sent: Monday, December 03, 2007 9:25 PM To: Raj Patel Cc: Milazzo Giacomo; toasters@mathworks.com Subject: RE: Can't expand SnapDrive for Windows drive
You can lower it. vol options <volname> fractional_reserve xx
-----Original Message----- From: Raj Patel [mailto:phigmov@gmail.com] Sent: Monday, December 03, 2007 1:02 PM To: Holland, William L Cc: Milazzo Giacomo; toasters@mathworks.com Subject: Re: Can't expand SnapDrive for Windows drive
The default is to have a volume at least twice the size of a LUN to
accomodate at least one full snapshot.
I have heard the new versions of DataOntap and SnapDrive allow for a lower ratio - I'm guessing you can't retrospectively upgrade and reduce the space allocation on existing volumes ?
Or have I misheard the capabilities of the new versions ?
Cheers, Raj.