Fractional Reserve, SNapshots and Space Usage

allanblack — Thu, 05 Jun 2025 06:08:29 GMT

I thought I understood fractional reserve 🙂 until I came across this case, which seems to be an anomaly to me. Is anyone able to explain how the disk space is being used in this example?

Consider two volumes: volproddata03 and volproddiag.

They are both the same size:

% ssh filer df -rk | egrep '(proddiag|proddata03)'
Filesystem total used avail reserved

/vol/volproddata03/ 5033168KB 3357368KB 1675800KB 1258728KB /vol/volproddata03/

snap reserve 1258288KB 772800KB 485488KB 0KB /vol/volproddata03/..

/vol/volproddiag/ 5033168KB 4193344KB 839824KB 2094868KB /vol/volproddiag/

snap reserve 1258288KB 149024KB 1109264KB 0KB /vol/volproddiag/..

but volproddiag is using much more space than volproddata03, despite the fact that the snapshots are smaller:

% ssh filer snap list volproddata03

Volume volproddata03

working...

%/used %/total date name

---------- ---------- ------------ --------

23% (23%) 6% ( 6%) Mar 05 00:55 snproddata03.1

38% (23%) 12% ( 6%) Mar 04 00:55 snproddata03.2

%ssh filer snap list volproddiag

Volume volproddiag

working...

%/used %/total date name

---------- ---------- ------------ --------

3% ( 3%) 1% ( 1%) Mar 05 00:55 snproddiag.1

7% ( 4%) 2% ( 1%) Mar 04 00:55 snproddiag.2

The fractional reserve for both volumes is set to 100% and the guarantee is set to 'volume' in both cases.

Both volumes also contain one LUN each, both the same size, and both LUNs are space reserved.

%ssh filer lun show /vol/volproddiag/volproddiaglun

/vol/volproddiag/volproddiaglun 2.0g (2142240768) (r/w, online, mapped)

%ssh filer lun show /vol/volproddata03/volproddata03lun

/vol/volproddata03/volproddata03lun 2.0g (2142240768) (r/w, online, mapped)

I cannot see any reason why the disk usage for volproddiag (83%) should be higher than volproddata03 (67%)

%ssh filer df -k | egrep '(proddiag|proddata03)'

Filesystem total used avail capacity Mounted on

/vol/volproddata03/ 5033168KB 3357516KB 1675652KB 67% /vol/volproddata03/

snap reserve 1258288KB 772812KB 485476KB 61% /vol/volproddata03/..

/vol/volproddiag/ 5033168KB 4193344KB 839824KB 83% /vol/volproddiag/

snap reserve 1258288KB 149040KB 1109248KB 12% /vol/volproddiag/..

The significant numbers seem to be the 'reserved' column in the 'df -rk' output. However, I cannot see the reason why the reserved space should be so much greater for the volproddiag volume. Is anyone able to explain why?

Re: Fractional Reserve, SNapshots and Space Usage

radek_kubka — Tue, 05 Mar 2013 18:13:43 GMT

Hi and welcome to the Community!

I reckon one of the LUNs is filled with more data - it would be more obvious if LUNs are not space reserved.

Fractional reserve takes into consideration only blocks actually used by LUN, even if the LUN itself is space reserved.

Starting with ONTAP 8, you can see actual LUN usage in lun show -v output.

Regards,

Radek

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Fractional Reserve, SNapshots and Space Usage

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