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There can be more than one DVA because you may have set the copies property to 2 or 3, or this may be metadata (which normally has two copies and may have more for sufficiently important metadata). Up to three DVAs that say where to actually find the data on disk.various metadata and flags about what the block pointer is for and what parts of it mean, including what type of object it points to.So what’s in a block pointer itself? You can find the technical details for modern ZFS in spa.h, so I’m going to give a sort of summary.
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However, as I discovered when I paid attention, most things in ZFS only point to dnodes indirectly, by giving their object number (either in a ZFS filesystem or in pool-wide metadata). For instance, the dnode for a file contains block pointers that refer to either its data blocks (if it’s small enough) or indirect blocks, as I saw in this entry. To quote from the (draft and old) ZFS on-disk specification (PDF):Ī block pointer (blkptr_t) is a 128 byte ZFS structure used to physically locate, verify, and describe blocks of data on disk.īlock pointers are embedded in any ZFS on disk structure that points directly to other disk blocks, both for data and metadata. Just like block numbers but unlike things like ZFS dnodes, a block pointer isn’t a separate on-disk entity instead it’s an on disk data format and an in memory structure that shows up in other things. The very simple way to describe a ZFS block pointer is that it’s what ZFS uses in places where other filesystems would simply put a block number. But I’ve never really looked carefully at what is in block pointers and what that means and implies for ZFS.
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I’ve mentioned ZFS block pointers in the past for example, when I wrote about some details of ZFS DVAs, I said that DVAs are embedded in block pointers. # What ZFS block pointers are and what’s in them What ZFS blockpointers are, zero-day rewards offered, KDE on FreeBSD status, new FreeBSD core team, NetBSD WiFi refresh, poor man’s CI, and the power of Ctrl+T.