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From: Andriy Gapon <avg@FreeBSD.org>
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To: Adam Nowacki <nowakpl@platinum.linux.pl>
Subject: Re: ZFS slow reads for unallocated blocks
References: <5166EA43.7050700@platinum.linux.pl>
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ENOTIME to really investigate, but here is a basic profile result for those
interested:
              kernel`bzero+0xa
              kernel`dmu_buf_hold_array_by_dnode+0x1cf
              kernel`dmu_read_uio+0x66
              kernel`zfs_freebsd_read+0x3c0
              kernel`VOP_READ_APV+0x92
              kernel`vn_read+0x1a3
              kernel`vn_io_fault+0x23a
              kernel`dofileread+0x7b
              kernel`sys_read+0x9e
              kernel`amd64_syscall+0x238
              kernel`0xffffffff80747e4b

That's where > 99% of time is spent.
-- 
Andriy Gapon