Solaris UFS full while df shows plen­ty of free space/inodes

At work we have a Solaris 8 with a UFS which told the appli­ca­tion that it can not cre­ate new files. The df com­mand showed plen­ty if free inodes, and there was also enough space free in the FS. The rea­son that the appli­ca­tion got the error was that while there where still plen­ty of frag­ments free, no free block was avail­able any­more. You can not cre­ate a new file only with frag­ments, you need to have at least one free block for each new file.

To see the num­ber of free blocks of a UFS you can call “fstyp ‑v | head ‑18” and look at the val­ue behind “nbfree”.

To get this work­ing again we cleaned up the FS a lit­tle bit (compressing/deleting log files), but this is only a tem­po­rary solu­tion. Unluck­i­ly we can not move this appli­ca­tion to a Solaris 10 with ZFS, so I was play­ing around a lit­tle bit to see what we can do.

First I made a his­togram of the file sizes. The back­up of the FS I was play­ing with had a lit­tle bit more than 4 mil­lion files in this FS. 28.5% of them where small­er than or equal 512 bytes, 31.7% where small­er than or equal 1k (frag­ment size), 36% small­er than or equal 8k (block size) and 74% small­er than or equal 16k. The fol­low­ing graph shows in red the crit­i­cal part, files which need a block and pro­duce frag­ments, but can not life with only frag­ments.

chart

Then I played around with newfs options for this one spe­cif­ic FS with this spe­cif­ic data mix. Chang­ing the num­ber of inodes did not change much the out­come for our prob­lem (as expect­ed). Chang­ing the opti­miza­tion from “time” to “space” (and restor­ing all the data from back­up into the emp­ty FS) gave us 1000 more free blocks. On a FS which had 10 Mio free blocks when emp­ty this is not much, but we expect that the restore con­sumes less frag­ments and more full blocks than the live-FS of the appli­ca­tion (we can not com­pare, as the con­tent of the live-FS changed a lot since we had the prob­lem). We assume that e.g. the logs of the appli­ca­tion are split over a lot of frag­ments instead of full blocks, due to small writes to the logs by the appli­ca­tion. The restore should write all the data in big chunks, so our expec­ta­tion is that the FS will use more full blocks and less frag­ments. Because of this we expect that the live-FS with this spe­cif­ic data mix could ben­e­fit from chang­ing the opti­miza­tion.

I also played around with the frag­ment size. The expec­ta­tion was that it will only change what is report­ed in the out­put of df (reduc­ing the report­ed avail­able space for the same amount of data). Here is the result:

chart

The dif­fer­ence between 1k (default) and 2k is not much. For 8k we would have to much unused space lost. The frag­ment size of 4k looks like it is accept­able to get a bet­ter mon­i­tor­ing sta­tus of this par­tic­u­lar data mix.

Based upon this we will prob­a­bly cre­ate a new FS with a frag­ment size of 4k and we will prob­a­bly switch the opti­miza­tion direct­ly to “space”. This way we will have a bet­ter report­ing on the fill lev­el of the FS for our data mix (but we will not be able to ful­ly use the real space of the FS) and as such our mon­i­tor­ing should alert us in time to do a cleanup of the FS or to increase the size of the FS.

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