Date: Sun, 17 Mar 2019 09:58:52 -0500 From: Karl Denninger <karl@denninger.net> To: freebsd-stable@freebsd.org Subject: Observations from a ZFS reorganization on 12-STABLE Message-ID: <58eb1994-41bd-cd22-be66-0024bcbc36e6@denninger.net>
next in thread | raw e-mail | index | archive | help
This is a cryptographically signed message in MIME format. --------------ms020602020202040801080604 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable I've long argued that the VM system's interaction with ZFS' arc cache and UMA has serious, even severe issues.=C2=A0 12.x appeared to have addressed some of them, and as such I've yet to roll forward any part of the patch series that is found here [ https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D187594 ] or the Phabricator version referenced in the bug thread (which is more-complex and attempts to dig at the root of the issue more effectively, particularly when UMA is involved as it usually is.) Yesterday I decided to perform a fairly significant reorganization of the ZFS pools on one of my personal machines, including the root pool which was on mirrored SSDs, changing to a Raidz2 (also on SSDs.)=C2=A0 Th= is of course required booting single-user from a 12-Stable memstick. A simple "zfs send -R zs/root-save/R | zfs recv -Fuev zsr/R" should have done it, no sweat.=C2=A0 The root that was copied over before I started i= s uncomplicated; it's compressed, but not de-duped.=C2=A0 While it has snapshots on it too it's by no means complex. *The system failed to execute that command with an "out of swap space" error, killing the job; there was indeed no swap configured since I booted from a memstick.* Huh?=C2=A0 A simple *filesystem copy* managed to force a 16Gb system into= requiring page file backing store? I was able to complete the copy by temporarily adding the swap space back on (where it would be when the move was complete) but that requirement is pure insanity and it appears, from what I was able to determine, that it came about from the same root cause that's been plaguing VM/ZFS interaction since 2014 when I started work this issue -- specifically, when RAM gets low rather than evict ARC (or clean up UMA that is allocated but unused) the system will attempt to page out working set.=C2=A0 In this case since it couldn't page out working set si= nce there was nowhere to page it to the process involved got an OOM error and was terminated. *I continue to argue that this decision is ALWAYS wrong.* It's wrong because if you invalidate cache and reclaim it you *might* take a read from physical I/O to replace that data back into the cache in the future (since it's not in RAM) but in exchange for a *potential* I/O you perform a GUARANTEED physical I/O (to page out some amount of working set) and possibly TWO physical I/Os (to page said working set out and, later, page it back in.) It has always appeared to me to be flat-out nonsensical to trade a possible physical I/O (if there is a future cache miss) for a guaranteed physical I/O and a possible second one.=C2=A0 It's even worse if the reas= on you make that decision is that UMA is allocated but unused; in that case you are paging when no physical I/O is required at all as the "memory pressure" is a phantom!=C2=A0 While UMA is a very material performance wi= n in the general case to allow allocated-but-unused UMA to force paging, from a performance perspective, appears to be flat-out insanity.=C2=A0 I find = it very difficult to come up with any reasonable scenario where releasing allocated-but-unused UMA rather than paging out working set is a net performance loser. In this case since the system was running in single user mode the process that got selected to be destroyed when that circumstance arose and there was no available swap was the copy process itself.=C2=A0 The co= py itself did not require anywhere near all of the available non-kernel RAM.= I'm going to dig into this further but IMHO the base issue still exists, even though the impact of it for my workloads with everything "running normally" has materially decreased with 12.x. --=20 Karl Denninger karl@denninger.net <mailto:karl@denninger.net> /The Market Ticker/ /[S/MIME encrypted email preferred]/ --------------ms020602020202040801080604 Content-Type: application/pkcs7-signature; name="smime.p7s" Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename="smime.p7s" Content-Description: S/MIME Cryptographic Signature MIAGCSqGSIb3DQEHAqCAMIACAQExDzANBglghkgBZQMEAgMFADCABgkqhkiG9w0BBwEAAKCC DdgwggagMIIEiKADAgECAhMA5EiKghDOXrvfxYxjITXYDdhIMA0GCSqGSIb3DQEBCwUAMIGL MQswCQYDVQQGEwJVUzEQMA4GA1UECAwHRmxvcmlkYTESMBAGA1UEBwwJTmljZXZpbGxlMRkw FwYDVQQKDBBDdWRhIFN5c3RlbXMgTExDMRgwFgYDVQQLDA9DdWRhIFN5c3RlbXMgQ0ExITAf BgNVBAMMGEN1ZGEgU3lzdGVtcyBMTEMgMjAxNyBDQTAeFw0xNzA4MTcxNjQyMTdaFw0yNzA4 MTUxNjQyMTdaMHsxCzAJBgNVBAYTAlVTMRAwDgYDVQQIDAdGbG9yaWRhMRkwFwYDVQQKDBBD dWRhIFN5c3RlbXMgTExDMRgwFgYDVQQLDA9DdWRhIFN5c3RlbXMgQ0ExJTAjBgNVBAMMHEN1 ZGEgU3lzdGVtcyBMTEMgMjAxNyBJbnQgQ0EwggIiMA0GCSqGSIb3DQEBAQUAA4ICDwAwggIK AoICAQC1aJotNUI+W4jP7xQDO8L/b4XiF4Rss9O0B+3vMH7Njk85fZ052QhZpMVlpaaO+sCI KqG3oNEbuOHzJB/NDJFnqh7ijBwhdWutdsq23Ux6TvxgakyMPpT6TRNEJzcBVQA0kpby1DVD 0EKSK/FrWWBiFmSxg7qUfmIq/mMzgE6epHktyRM3OGq3dbRdOUgfumWrqHXOrdJz06xE9NzY vc9toqZnd79FUtE/nSZVm1VS3Grq7RKV65onvX3QOW4W1ldEHwggaZxgWGNiR/D4eosAGFxn uYeWlKEC70c99Mp1giWux+7ur6hc2E+AaTGh+fGeijO5q40OGd+dNMgK8Es0nDRw81lRcl24 SWUEky9y8DArgIFlRd6d3ZYwgc1DMTWkTavx3ZpASp5TWih6yI8ACwboTvlUYeooMsPtNa9E 6UQ1nt7VEi5syjxnDltbEFoLYcXBcqhRhFETJe9CdenItAHAtOya3w5+fmC2j/xJz29og1KH YqWHlo3Kswi9G77an+zh6nWkMuHs+03DU8DaOEWzZEav3lVD4u76bKRDTbhh0bMAk4eXriGL h4MUoX3Imfcr6JoyheVrAdHDL/BixbMH1UUspeRuqQMQ5b2T6pabXP0oOB4FqldWiDgJBGRd zWLgCYG8wPGJGYgHibl5rFiI5Ix3FQncipc6SdUzOQIDAQABo4IBCjCCAQYwHQYDVR0OBBYE FF3AXsKnjdPND5+bxVECGKtc047PMIHABgNVHSMEgbgwgbWAFBu1oRhUMNEzjODolDka5k4Q EDBioYGRpIGOMIGLMQswCQYDVQQGEwJVUzEQMA4GA1UECAwHRmxvcmlkYTESMBAGA1UEBwwJ TmljZXZpbGxlMRkwFwYDVQQKDBBDdWRhIFN5c3RlbXMgTExDMRgwFgYDVQQLDA9DdWRhIFN5 c3RlbXMgQ0ExITAfBgNVBAMMGEN1ZGEgU3lzdGVtcyBMTEMgMjAxNyBDQYIJAKxAy1WBo2kY MBIGA1UdEwEB/wQIMAYBAf8CAQAwDgYDVR0PAQH/BAQDAgGGMA0GCSqGSIb3DQEBCwUAA4IC AQCB5686UCBVIT52jO3sz9pKuhxuC2npi8ZvoBwt/IH9piPA15/CGF1XeXUdu2qmhOjHkVLN gO7XB1G8CuluxofOIUce0aZGyB+vZ1ylHXlMeB0R82f5dz3/T7RQso55Y2Vog2Zb7PYTC5B9 oNy3ylsnNLzanYlcW3AAfzZcbxYuAdnuq0Im3EpGm8DoItUcf1pDezugKm/yKtNtY6sDyENj tExZ377cYA3IdIwqn1Mh4OAT/Rmh8au2rZAo0+bMYBy9C11Ex0hQ8zWcvPZBDn4v4RtO8g+K uQZQcJnO09LJNtw94W3d2mj4a7XrsKMnZKvm6W9BJIQ4Nmht4wXAtPQ1xA+QpxPTmsGAU0Cv HmqVC7XC3qxFhaOrD2dsvOAK6Sn3MEpH/YrfYCX7a7cz5zW3DsJQ6o3pYfnnQz+hnwLlz4MK 17NIA0WOdAF9IbtQqarf44+PEyUbKtz1r0KGeGLs+VGdd2FLA0e7yuzxJDYcaBTVwqaHhU2/ Fna/jGU7BhrKHtJbb/XlLeFJ24yvuiYKpYWQSSyZu1R/gvZjHeGb344jGBsZdCDrdxtQQcVA 6OxsMAPSUPMrlg9LWELEEYnVulQJerWxpUecGH92O06wwmPgykkz//UmmgjVSh7ErNvL0lUY UMfunYVO/O5hwhW+P4gviCXzBFeTtDZH259O7TCCBzAwggUYoAMCAQICEwCg0WvVwekjGFiO 62SckFwepz0wDQYJKoZIhvcNAQELBQAwezELMAkGA1UEBhMCVVMxEDAOBgNVBAgMB0Zsb3Jp ZGExGTAXBgNVBAoMEEN1ZGEgU3lzdGVtcyBMTEMxGDAWBgNVBAsMD0N1ZGEgU3lzdGVtcyBD QTElMCMGA1UEAwwcQ3VkYSBTeXN0ZW1zIExMQyAyMDE3IEludCBDQTAeFw0xNzA4MTcyMTIx MjBaFw0yMjA4MTYyMTIxMjBaMFcxCzAJBgNVBAYTAlVTMRAwDgYDVQQIDAdGbG9yaWRhMRkw FwYDVQQKDBBDdWRhIFN5c3RlbXMgTExDMRswGQYDVQQDDBJrYXJsQGRlbm5pbmdlci5uZXQw ggIiMA0GCSqGSIb3DQEBAQUAA4ICDwAwggIKAoICAQC+HVSyxVtJhy3Ohs+PAGRuO//Dha9A 16l5FPATr6wude9zjX5f2lrkRyU8vhCXTZW7WbvWZKpcZ8r0dtZmiK9uF58Ec6hhvfkxJzbg 96WHBw5Fumd5ahZzuCJDtCAWW8R7/KN+zwzQf1+B3MVLmbaXAFBuKzySKhKMcHbK3/wjUYTg y+3UK6v2SBrowvkUBC+jxNg3Wy12GsTXcUS/8FYIXgVVPgfZZrbJJb5HWOQpvvhILpPCD3xs YJFNKEPltXKWHT7Qtc2HNqikgNwj8oqOb+PeZGMiWapsatKm8mxuOOGOEBhAoTVTwUHlMNTg 6QUCJtuWFCK38qOCyk9Haj+86lUU8RG6FkRXWgMbNQm1mWREQhw3axgGLSntjjnznJr5vsvX SYR6c+XKLd5KQZcS6LL8FHYNjqVKHBYM+hDnrTZMqa20JLAF1YagutDiMRURU23iWS7bA9tM cXcqkclTSDtFtxahRifXRI7Epq2GSKuEXe/1Tfb5CE8QsbCpGsfSwv2tZ/SpqVG08MdRiXxN 5tmZiQWo15IyWoeKOXl/hKxA9KPuDHngXX022b1ly+5ZOZbxBAZZMod4y4b4FiRUhRI97r9l CxsP/EPHuuTIZ82BYhrhbtab8HuRo2ofne2TfAWY2BlA7ExM8XShMd9bRPZrNTokPQPUCWCg CdIATQIDAQABo4IBzzCCAcswPAYIKwYBBQUHAQEEMDAuMCwGCCsGAQUFBzABhiBodHRwOi8v b2NzcC5jdWRhc3lzdGVtcy5uZXQ6ODg4ODAJBgNVHRMEAjAAMBEGCWCGSAGG+EIBAQQEAwIF oDAOBgNVHQ8BAf8EBAMCBeAwHQYDVR0lBBYwFAYIKwYBBQUHAwIGCCsGAQUFBwMEMDMGCWCG SAGG+EIBDQQmFiRPcGVuU1NMIEdlbmVyYXRlZCBDbGllbnQgQ2VydGlmaWNhdGUwHQYDVR0O BBYEFLElmNWeVgsBPe7O8NiBzjvjYnpRMIHKBgNVHSMEgcIwgb+AFF3AXsKnjdPND5+bxVEC GKtc047PoYGRpIGOMIGLMQswCQYDVQQGEwJVUzEQMA4GA1UECAwHRmxvcmlkYTESMBAGA1UE BwwJTmljZXZpbGxlMRkwFwYDVQQKDBBDdWRhIFN5c3RlbXMgTExDMRgwFgYDVQQLDA9DdWRh IFN5c3RlbXMgQ0ExITAfBgNVBAMMGEN1ZGEgU3lzdGVtcyBMTEMgMjAxNyBDQYITAORIioIQ zl6738WMYyE12A3YSDAdBgNVHREEFjAUgRJrYXJsQGRlbm5pbmdlci5uZXQwDQYJKoZIhvcN AQELBQADggIBAJXboPFBMLMtaiUt4KEtJCXlHO/3ZzIUIw/eobWFMdhe7M4+0u3te0sr77QR dcPKR0UeHffvpth2Mb3h28WfN0FmJmLwJk+pOx4u6uO3O0E1jNXoKh8fVcL4KU79oEQyYkbu 2HwbXBU9HbldPOOZDnPLi0whi/sbFHdyd4/w/NmnPgzAsQNZ2BYT9uBNr+jZw4SsluQzXG1X lFL/qCBoi1N2mqKPIepfGYF6drbr1RnXEJJsuD+NILLooTNf7PMgHPZ4VSWQXLNeFfygoOOK FiO0qfxPKpDMA+FHa8yNjAJZAgdJX5Mm1kbqipvb+r/H1UAmrzGMbhmf1gConsT5f8KU4n3Q IM2sOpTQe7BoVKlQM/fpQi6aBzu67M1iF1WtODpa5QUPvj1etaK+R3eYBzi4DIbCIWst8MdA 1+fEeKJFvMEZQONpkCwrJ+tJEuGQmjoQZgK1HeloepF0WDcviiho5FlgtAij+iBPtwMuuLiL shAXA5afMX1hYM4l11JXntle12EQFP1r6wOUkpOdxceCcMVDEJBBCHW2ZmdEaXgAm1VU+fnQ qS/wNw/S0X3RJT1qjr5uVlp2Y0auG/eG0jy6TT0KzTJeR9tLSDXprYkN2l/Qf7/nT6Q03qyE QnnKiBXWAZXveafyU/zYa7t3PTWFQGgWoC4w6XqgPo4KV44OMYIFBzCCBQMCAQEwgZIwezEL MAkGA1UEBhMCVVMxEDAOBgNVBAgMB0Zsb3JpZGExGTAXBgNVBAoMEEN1ZGEgU3lzdGVtcyBM TEMxGDAWBgNVBAsMD0N1ZGEgU3lzdGVtcyBDQTElMCMGA1UEAwwcQ3VkYSBTeXN0ZW1zIExM QyAyMDE3IEludCBDQQITAKDRa9XB6SMYWI7rZJyQXB6nPTANBglghkgBZQMEAgMFAKCCAkUw GAYJKoZIhvcNAQkDMQsGCSqGSIb3DQEHATAcBgkqhkiG9w0BCQUxDxcNMTkwMzE3MTQ1ODUy WjBPBgkqhkiG9w0BCQQxQgRA1Hbp7Lq00nbsTPSM58W2vssmS/Lu4m5rbhg/Q/XVJqgYEiQ4 XEjgZKH1aw9WyoOOSbH4FZKzie7XBPMM6jQDBTBsBgkqhkiG9w0BCQ8xXzBdMAsGCWCGSAFl AwQBKjALBglghkgBZQMEAQIwCgYIKoZIhvcNAwcwDgYIKoZIhvcNAwICAgCAMA0GCCqGSIb3 DQMCAgFAMAcGBSsOAwIHMA0GCCqGSIb3DQMCAgEoMIGjBgkrBgEEAYI3EAQxgZUwgZIwezEL MAkGA1UEBhMCVVMxEDAOBgNVBAgMB0Zsb3JpZGExGTAXBgNVBAoMEEN1ZGEgU3lzdGVtcyBM TEMxGDAWBgNVBAsMD0N1ZGEgU3lzdGVtcyBDQTElMCMGA1UEAwwcQ3VkYSBTeXN0ZW1zIExM QyAyMDE3IEludCBDQQITAKDRa9XB6SMYWI7rZJyQXB6nPTCBpQYLKoZIhvcNAQkQAgsxgZWg gZIwezELMAkGA1UEBhMCVVMxEDAOBgNVBAgMB0Zsb3JpZGExGTAXBgNVBAoMEEN1ZGEgU3lz dGVtcyBMTEMxGDAWBgNVBAsMD0N1ZGEgU3lzdGVtcyBDQTElMCMGA1UEAwwcQ3VkYSBTeXN0 ZW1zIExMQyAyMDE3IEludCBDQQITAKDRa9XB6SMYWI7rZJyQXB6nPTANBgkqhkiG9w0BAQEF AASCAgA5zkDs51DPi/+B/Y0u2J7KnEA1ERUF9oexczq/lEYbTJlOxTdoUv0aAkuavEf2xfLi 9AadsywtM0BkWnOmIaR1NJdOiWye/n6oIvk/XhYk60yK+rkLZR5CojX20sOkOdEOobVnMsUm EA2nBTWPjT9XdeVgrwQrWvs7fza4tXsSm1238dtYX03ZWjRlyAtwVosSG4z42so7trCiVsLC j2QHl9RbC0RMwWTFZLlpT+c1n5Jx+ims4aMxAM5E6n3EfaqzlSIMXp9D+65XW/eFRcrg1aRy ujfYlOoU6PRQWNddAsut81I7gn2jNOVh7F9fEwb2hPhNS6ygoNW0JI6dOsLpZtuYELkRzDhc FvOPza78SIcAU/jLMWA8g+YB18t1MufqJgT3xEyFErcGHgoh5HQdWhHRnQu2ftC3nyF7WU0r r/75XM95p6qTh2u/bc1yfJ+i6WjAD1VFVvj8v/fvMhA60RgcPIjEzUlGnt8BEbrX5TElk3im grumWb2EUg3APkNcWu7qez2gvX41HfSFKZ1ol/Fjn9w8G79hdb8jV5c30cei6csGUZAVcFJG VhQwIhQ9LLoLJtIQ1vKrOPvSMogkGs/m2RhSKRDJMEcRcUNqfgvjBH6bDfSL+9HLmi2LVOS3 KqWpf4rgPr7ENGEr6EPJ4h84uOH3+FRkTInWz6j+NAAAAAAAAA== --------------ms020602020202040801080604--
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?58eb1994-41bd-cd22-be66-0024bcbc36e6>