diff --git a/README.md b/README.md
index 27459f1..a814bc2 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,64 @@
 # auto-rules-io
-automatic udev rules to io
+I/O, memory and boot tuning for BigLinux
 
-EN - Automatically choose input and output scheduler for BFQ on rotational disks, SSD and none for NVME.
+## I/O scheduler / Agendador de E/S
 
-PT - Escolher automaticamente o agendador de entrada e saída para BFQ em discos rotacionais, SSD e nenhum para NVME.
\ No newline at end of file
+EN - Automatically chooses the I/O scheduler: BFQ on rotational disks and eMMC, Kyber on SSD and NVMe.
+
+PT - Escolhe automaticamente o agendador de E/S: BFQ em discos rotacionais e eMMC, Kyber em SSD e NVMe.
+
+## Disk latency target for the session / Meta de latência de disco da sessão
+
+EN - The session keeps a 2 ms disk latency target on the disks of `/` and `/home` (`IODeviceLatencyTargetSec=` on `user.slice`, and on `app.slice` and `session.slice` in the user manager, which `Delegate=io` on `user@.service` allows). While programs miss it, the kernel's io.latency controller throttles everything beside them: `system.slice` and `machine.slice` (system services, containers and virtual machines, including work done for the user such as package updates and Docker builds) and the user's `background.slice` (indexers). While the session does no I/O, they run at full speed. This works under any scheduler: ionice classes act only inside one cgroup under BFQ and not at all under Kyber, and `IOWeight` shares throughput under BFQ but bounds no latency.
+
+PT - A sessão mantém uma meta de latência de 2 ms nos discos de `/` e `/home` (`IODeviceLatencyTargetSec=` em `user.slice`, e em `app.slice` e `session.slice` no gerenciador do usuário, o que o `Delegate=io` no `user@.service` permite). Enquanto os programas passam dessa meta, o controlador io.latency do kernel freia tudo o que roda ao lado deles: `system.slice` e `machine.slice` (serviços do sistema, contêineres e máquinas virtuais, inclusive trabalho feito para o usuário, como atualizações de pacotes e builds do Docker) e a `background.slice` do usuário (indexadores). Enquanto a sessão não faz E/S, eles rodam na velocidade máxima. Funciona com qualquer agendador: as classes do ionice só valem dentro de um cgroup no BFQ e não valem no Kyber, e o `IOWeight` divide a vazão no BFQ, mas não limita a latência.
+
+EN - All I/O from the user's session counts toward the `user.slice` target, `background.slice` included: while the reads of an indexer such as Baloo or localsearch miss the target, system services and VMs are throttled even if no program waits. The kernel adds each request's latency to every ancestor group that has a target, and `background.slice` lives inside `user.slice`.
+
+PT - Toda E/S da sessão do usuário conta para a meta da `user.slice`, inclusive a da `background.slice`: enquanto as leituras de um indexador como o Baloo ou o localsearch passam da meta, serviços do sistema e VMs são freados mesmo sem nenhum programa esperando. O kernel soma a latência de cada requisição em todos os grupos ancestrais que têm meta, e a `background.slice` fica dentro da `user.slice`.
+
+EN - Measured with a sequential reader in the background (cold start, seconds). Alone, LibreOffice and Firefox start in 0.8 and 1.4 s on the NVMe, 10.6 and 16.5 s on the HDD.
+
+PT - Medido com um leitor sequencial em segundo plano (início a frio, segundos). Sozinhos, LibreOffice e Firefox abrem em 0,8 e 1,4 s no NVMe e em 10,6 e 16,5 s no HD.
+
+| Disk | Target | LibreOffice | Firefox |
+|---|---|---|---|
+| NVMe | none | 16.5–45 | 65–71 |
+| NVMe | 2 ms | 1.5–2.4 | 2.1–3.2 |
+| HDD | none | 104–111 | 112–132 |
+| HDD | 2 ms | 37–40 | 74–80 |
+
+EN - On the NVMe, a reader that saturates the disk loses about a quarter of its rate while the session does any I/O; buffered writes and random reads lose nothing measurable, and nothing measurable is lost on the HDD. On an HDD the target does not help against buffered writes, whose writeback it does not control.
+
+PT - No NVMe, um leitor que satura o disco perde cerca de um quarto da vazão enquanto a sessão faz qualquer E/S; escritas em buffer e leituras aleatórias não perdem nada mensurável, e no HD nada mensurável se perde. No HD a meta não ajuda contra escritas em buffer, cujo writeback ela não controla.
+
+EN - The target was measured on plain partitions. systemd resolves a LUKS volume, or an LVM volume on one disk, to the disk under it. On RAID, LVM over several disks, multi-device Btrfs or a network `/home` the target cannot reach the disks and does not help.
+
+PT - A meta foi medida em partições comuns. O systemd resolve um volume LUKS, ou um volume LVM sobre um disco só, para o disco que está por baixo. Em RAID, LVM sobre vários discos, Btrfs com vários discos ou `/home` em rede, a meta não alcança os discos e não ajuda.
+
+EN - `user.slice` holds every login, SSH and root included. On a server, an administrator's shell would throttle databases, containers and VMs. To turn the target off, mask the three drop-ins and reboot:
+
+PT - A `user.slice` reúne todos os logins, inclusive SSH e root. Num servidor, o shell de um administrador frearia bancos de dados, contêineres e VMs. Para desligar a meta, mascare os três drop-ins e reinicie:
+
+```bash
+sudo mkdir -p /etc/systemd/system/user.slice.d /etc/systemd/user/app.slice.d /etc/systemd/user/session.slice.d
+sudo ln -s /dev/null /etc/systemd/system/user.slice.d/50-io-latency.conf
+sudo ln -s /dev/null /etc/systemd/user/app.slice.d/50-io-latency.conf
+sudo ln -s /dev/null /etc/systemd/user/session.slice.d/50-io-latency.conf
+```
+
+EN - Removing the target takes effect at the next boot: systemd drops the setting but leaves the kernel's target in place. Clearing it by hand (`target=0` in `io.latency`) while it throttles leaves the throttled groups delayed until reboot, since the kernel clears the delay only on the group that had the target.
+
+PT - Remover a meta só vale no próximo boot: o systemd descarta a configuração, mas deixa a meta no kernel. Zerá-la à mão (`target=0` em `io.latency`) enquanto ela freia deixa os grupos freados atrasados até reiniciar, porque o kernel só limpa o atraso do grupo que tinha a meta.
+
+## Linker cache at boot / Cache do ligador no boot
+
+EN - `ldconfig.service` rebuilds `/etc/ld.so.cache` at boot only when it is missing or empty, instead of at the first boot after every update. pacman already runs ldconfig after each transaction, so the boot rebuild repeated it inside `sysinit.target`: 8.8–9.9 s cold on an HDD, 0.5 s on an NVMe. Libraries added without pacman (a `systemd-sysext` image, a transaction killed before its end) need `sudo ldconfig`.
+
+PT - O `ldconfig.service` refaz o `/etc/ld.so.cache` no boot só quando ele falta ou está vazio, em vez de no primeiro boot depois de cada atualização. O pacman já roda o ldconfig a cada transação, então o boot repetia o trabalho dentro do `sysinit.target`: 8,8–9,9 s a frio num HD, 0,5 s num NVMe. Bibliotecas adicionadas sem o pacman (uma imagem do `systemd-sysext`, uma transação interrompida antes do fim) precisam de `sudo ldconfig`.
+
+## snap at login / snap no login
+
+EN - snapd's autostart (`snap userd --autostart`) starts only where snapd is enabled (`/run/snapd.socket` exists). With snapd turned off it ran at every login: 3.3–4.2 s cold on an HDD.
+
+PT - O autostart do snapd (`snap userd --autostart`) só inicia onde o snapd está habilitado (existe `/run/snapd.socket`). Com o snapd desligado, ele rodava em todo login: 3,3–4,2 s a frio num HD.
diff --git a/auto-rules-io/usr/lib/systemd/system/ldconfig.service.d/50-pacman.conf b/auto-rules-io/usr/lib/systemd/system/ldconfig.service.d/50-pacman.conf
new file mode 100644
index 0000000..446637d
--- /dev/null
+++ b/auto-rules-io/usr/lib/systemd/system/ldconfig.service.d/50-pacman.conf
@@ -0,0 +1,8 @@
+# pacman runs ldconfig after every transaction (libalpm), so the cache is
+# current at boot; rebuilding it after each update only delays
+# sysinit.target. It is still built when missing or empty.
+# An empty assignment clears every condition of the unit, so the
+# missing-cache one is restated.
+[Unit]
+ConditionNeedsUpdate=
+ConditionFileNotEmpty=!/etc/ld.so.cache
diff --git a/auto-rules-io/usr/lib/systemd/system/user.slice.d/50-io-latency.conf b/auto-rules-io/usr/lib/systemd/system/user.slice.d/50-io-latency.conf
new file mode 100644
index 0000000..40d29ad
--- /dev/null
+++ b/auto-rules-io/usr/lib/systemd/system/user.slice.d/50-io-latency.conf
@@ -0,0 +1,12 @@
+# Disk latency target for the sessions on the system disks. While the
+# session's I/O misses it, the kernel (io.latency) throttles the other
+# top-level groups on that disk: system.slice and machine.slice, so system
+# services, containers and VMs, including work done for the user (package
+# updates, Docker builds). While the session does no I/O they run at full
+# speed. The user's background.slice is part of the session here: its I/O
+# counts toward this target too.
+# 2 ms is the one value that serves both NVMe and HDD: 1 ms is slower on
+# HDDs, 3 ms on NVMe.
+[Slice]
+IODeviceLatencyTargetSec=/ 2ms
+IODeviceLatencyTargetSec=/home 2ms
diff --git a/auto-rules-io/usr/lib/systemd/system/user@.service.d/50-io-latency.conf b/auto-rules-io/usr/lib/systemd/system/user@.service.d/50-io-latency.conf
new file mode 100644
index 0000000..9b9ba3f
--- /dev/null
+++ b/auto-rules-io/usr/lib/systemd/system/user@.service.d/50-io-latency.conf
@@ -0,0 +1,4 @@
+# The user manager sets the latency target of app.slice and session.slice,
+# which needs the io controller. systemd delegates only cpu, memory and pids.
+[Service]
+Delegate=io
diff --git "a/auto-rules-io/usr/lib/systemd/user/app-snap\\x2duserd\\x2dautostart@autostart.service.d/50-snapd.conf" "b/auto-rules-io/usr/lib/systemd/user/app-snap\\x2duserd\\x2dautostart@autostart.service.d/50-snapd.conf"
new file mode 100644
index 0000000..28a5277
--- /dev/null
+++ "b/auto-rules-io/usr/lib/systemd/user/app-snap\\x2duserd\\x2dautostart@autostart.service.d/50-snapd.conf"
@@ -0,0 +1,4 @@
+# snapd's XDG autostart (snap userd --autostart) has nothing to do where
+# snapd is turned off: start it only while snapd's socket exists.
+[Unit]
+ConditionPathExists=/run/snapd.socket
diff --git a/auto-rules-io/usr/lib/systemd/user/app.slice.d/50-io-latency.conf b/auto-rules-io/usr/lib/systemd/user/app.slice.d/50-io-latency.conf
new file mode 100644
index 0000000..c46097c
--- /dev/null
+++ b/auto-rules-io/usr/lib/systemd/user/app.slice.d/50-io-latency.conf
@@ -0,0 +1,6 @@
+# The same target as user.slice, one level down: while programs miss it, the
+# kernel throttles background.slice (indexers, backups). app.slice and
+# session.slice share the value, so neither throttles the other.
+[Slice]
+IODeviceLatencyTargetSec=/ 2ms
+IODeviceLatencyTargetSec=/home 2ms
diff --git a/auto-rules-io/usr/lib/systemd/user/session.slice.d/50-io-latency.conf b/auto-rules-io/usr/lib/systemd/user/session.slice.d/50-io-latency.conf
new file mode 100644
index 0000000..c46097c
--- /dev/null
+++ b/auto-rules-io/usr/lib/systemd/user/session.slice.d/50-io-latency.conf
@@ -0,0 +1,6 @@
+# The same target as user.slice, one level down: while programs miss it, the
+# kernel throttles background.slice (indexers, backups). app.slice and
+# session.slice share the value, so neither throttles the other.
+[Slice]
+IODeviceLatencyTargetSec=/ 2ms
+IODeviceLatencyTargetSec=/home 2ms
diff --git a/pkgbuild/PKGBUILD b/pkgbuild/PKGBUILD
index f83f7eb..d804b5f 100644
--- a/pkgbuild/PKGBUILD
+++ b/pkgbuild/PKGBUILD
@@ -6,7 +6,7 @@ pkgrel=$(date +%H%M)
 arch=('any')
 license=('GPL')
 url="https://github.com/biglinux/auto-rules-io"
-pkgdesc="Automatic change io scheduler"
+pkgdesc="BigLinux I/O, memory and boot tuning"
 source=("git+https://github.com/biglinux/auto-rules-io.git")
 makedepends=("git")
 md5sums=(SKIP)