Fractional Reserve, SNapshots and Space Usage

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


  %/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


  %/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

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.