On 2017/02/15 23:22, Tom Smyth wrote:
> Hello,
>
> I have been installing OpenBSD quite a bit in a virtualization
> environment and the
> underlying storage has been formatted in 1MB blocks, and most other operating
> systems have recommended for best performance to align the partition on a 1MB
> Offset rather than a smaller value.
>
> the reasoning behind this is that if you have files that are around
> than 1MB in size
> and they are stored in the file system it is likely that they will
> cross a block boundary and while OpenBSD only wants to read the file,
> the underlying storage system
> will have to retrieve 2 blocks and this is wasteful on resources.
>
> If the offset was set to 2048 sectors (at 1MB in my case) the disk
> performance of OpenBSD on my underlying system would be improved.
> I have been setting the value of the initial offset to 2048 and I have
> used exact sizes in-terms of sector count to keep my subsequent
> partitions aligned with the 1M offset,
> I have not noticed any ill affect on my installs (for the past 2
> years) and I think it
> would be worth considering this as a suggested default when installing OpenBSD
>
> Any comments suggestions insights welcome, as my experience on virtualization
> has been Vmware Based. But I would imagine most modern enterprise
> storage systems are using larger block sizes... and by setting it to a
> 1MB or perhaps a larger value (as long as the value ends on a 1MB
> Boundary eg 2MB or 4MB) depending on other users experience
>
I've been doing similar for installs on SSDs with their large erase blocks.
And this is common on other OS these days.
As you say, it's not just the starting point of the OpenBSD part of the disk,
care must be taken with the disklabel partitions too.
I have an old tree that has this diff in, I don't remember if it was quite
enough for disklabel as-is, but it might be enough of a starting point to
save time tracking things down.
Index: fdisk/mbr.c
===================================================================
RCS file: /cvs/src/sbin/fdisk/mbr.c,v
retrieving revision 1.67
diff -u -p -r1.67 mbr.c
--- fdisk/mbr.c 1 Sep 2016 16:17:46 -0000 1.67
+++ fdisk/mbr.c 15 Feb 2017 23:36:29 -0000
@@ -144,8 +144,11 @@ MBR_init(struct mbr *mbr)
}
#endif
- /* Start OpenBSD MBR partition on a power of 2 block number. */
- i = 1;
+ /*
+ * Start OpenBSD MBR partition on a power of 2 block number
+ * with a minimum 2048-block alignment.
+ */
+ i = 2048;
while (i < DL_SECTOBLK(&dl, mbr->part[3].bs))
i *= 2;
adj = DL_BLKTOSEC(&dl, i) - mbr->part[3].bs;
Index: disklabel/editor.c
===================================================================
RCS file: /cvs/src/sbin/disklabel/editor.c,v
retrieving revision 1.304
diff -u -p -r1.304 editor.c
--- disklabel/editor.c 6 Oct 2016 13:02:31 -0000 1.304
+++ disklabel/editor.c 15 Feb 2017 23:36:29 -0000
@@ -2092,8 +2092,7 @@ align:
orig_size = DL_GETPSIZE(pp);
orig_offset = DL_GETPOFFSET(pp);
- bsize = (DISKLABELV1_FFS_FRAG(pp->p_fragblock) *
- DISKLABELV1_FFS_FSIZE(pp->p_fragblock)) / lp->d_secsize;
+ bsize = 2048;
if (DL_GETPOFFSET(pp) != starting_sector) {
/* Can't change offset of first partition. */
adj = bsize - (DL_GETPOFFSET(pp) % bsize);
