On Mon, Jan 19, 2015 at 03:59:34AM +0000, currellbe...@gmail.com wrote: > Hello, > > The FAQ[1] states that soft updates result in "a large performance increase > in disk writing performance," and links to a resource[2] which claims that > soft updates, in addition to being a performance enhancement, "can also > maintain better disk consistency." Resource 2 links to several academic > papers[3][4], which while they are a bit above my level, contain discussions > of how soft updates can increase performance and speed recovery on crash. > > My question is: what are the downsides of soft updates?
- softdep consumes more cpu in kernel mode, which hurts interactive programms on very slow machines. It has the reputation of consuming more memory. - the softdep code is more complex (likely to have more bugs). > Also, does journaling provide a better data-safety guarantee? They are not the same. On OpenBSD, softdep makes cerain operations much faster while ensuring that upon power loss, all inconsistencies can be automatically fixed by fsck on next boot. Journaling would write data twice (first in the journal, then in the filesystem) and would allow last operations to be replayed on next boot, so no need to run fsck, which in turn makes system boot fast after a power loss. In theory, from data safety point of view they are equivalent.